Government Contract Invoicing — WAWF, Payment & Compliance
Getting paid on government contracts requires navigating specific invoicing systems, formats, and regulatory requirements. Improper invoices are the number one cause of delayed payments in federal contracting.
This guide covers the WAWF/iPPS invoicing system, invoice formats for each contract type, Prompt Payment Act protections, and the most common reasons invoices get rejected.
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WAWF / iPPS: The Invoicing System
Wide Area Workflow (WAWF), now part of the Invoicing, Receipt, Acceptance, and Property Transfer system (iPPS), is the Department of Defense's primary electronic invoicing system. Most DoD contracts require invoices to be submitted through WAWF. Civilian agencies typically use the Invoice Processing Platform (IPP) or agency-specific systems.
WAWF is accessed through the PIEE (Procurement Integrated Enterprise Environment) portal at piee.eb.mil. Before you can submit invoices, you need to register in PIEE, obtain the proper roles, and have your CAGE code and contract number ready. Registration can take 1-2 weeks, so set this up as soon as the contract is awarded — not when your first invoice is due.
Within WAWF, you will create invoice documents that correspond to your contract type. The system routes the invoice to the appropriate government acceptance official (typically the Contracting Officer's Representative) for review and approval, then forwards it to the Defense Finance and Accounting Service (DFAS) for payment processing.
PIEE Registration
Register at piee.eb.mil with your CAGE code. Request Vendor/Invoice roles. Allow 1-2 weeks for account activation.
Document Types
Select the correct document type: Invoice, Cost Voucher, Progress Payment, or Performance-Based Payment based on your contract.
Routing
Enter the correct DoDAAC codes for the issuing office, admin office, and pay office. Incorrect routing is the top cause of rejected invoices.
Invoice Types by Contract Type
Fixed-Price (FFP) Invoices
On firm-fixed-price contracts, you invoice based on deliverables completed or milestones achieved. The invoice amount corresponds to the fixed price for the delivered item or completed milestone as specified in the contract. FFP invoicing is the most straightforward: deliver the item, submit the invoice for the agreed price. Monthly invoices for services typically correspond to one month of the contract period of performance.
Cost-Reimbursement Invoices (Cost Vouchers)
Cost-reimbursement contracts require detailed cost vouchers that itemize all costs incurred during the billing period. These include direct labor (hours by labor category and rate), other direct costs (ODCs), travel, materials, subcontractor costs, and indirect costs applied at your provisional billing rates. Each cost must be allowable, allocable, and reasonable per FAR Part 31. Supporting documentation for individual transactions above $75 may be required.
Time & Materials (T&M) Invoices
T&M invoices include two components: labor hours at the fixed hourly rates specified in the contract, and materials at actual cost (plus any allowable material handling charges). Each invoice must list the employee name or labor category, hours worked, hourly rate, and extended amount. Materials must be itemized with supporting receipts. The government pays for actual hours worked, not estimated hours.
Progress Payments
Progress payments are interim payments made on fixed-price contracts before delivery, based on a percentage of costs incurred or a percentage of completion. They are governed by FAR 32.5 and require a progress payment request form (SF 1443). The standard progress payment rate is 80% of costs incurred for large businesses and 85% for small businesses. Progress payments are liquidated (subtracted) from delivery payments.
Prompt Payment Act Requirements
The Prompt Payment Act (31 U.S.C. §§ 3901-3907) requires the government to pay contractors within specific timeframes and to pay interest penalties on late payments. Understanding these requirements protects your cash flow and gives you recourse when payments are delayed.
Payment Deadlines
- • 30 days — Standard payment deadline from receipt of a proper invoice
- • 15 days — Meat, perishable agricultural commodities
- • 7 days — Acceptance period for inspecting goods (before the 30-day clock starts)
- • 14 days — For construction progress payments
Interest Penalties
- • Interest accrues automatically on late payments
- • Rate is set by the Treasury Department (current rate published semiannually)
- • Interest is calculated from the day after the payment due date
- • You do not need to request interest — the agency must pay it automatically
- • Interest penalties under $1 are not paid
The 30-day clock starts when the agency receives a "proper invoice" — one that contains all required information and is submitted correctly. If the agency determines your invoice is improper, they must notify you within 7 days of receipt, specifying the deficiency. The 30-day clock restarts when you submit the corrected invoice. This is why submitting a correct invoice the first time is critical for cash flow.
Common Invoice Rejection Reasons
The most frequent cause of rejection. Every contract has specific codes for the issuing office, admin office, inspector, and pay office. One wrong code and the invoice goes nowhere.
The invoice must reference the correct contract number, delivery/task order number, and CLIN. Typos or omissions cause immediate rejection.
Invoicing more than the currently funded amount on the contract will be rejected. Track incremental funding on cost-reimbursement and T&M contracts carefully.
Cost vouchers missing labor detail, T&M invoices missing timesheets, or material invoices missing receipts will be returned for correction.
Using indirect rates that differ from your approved provisional billing rates, or labor rates not matching the contract schedule, triggers rejection.
The COR must accept the services or deliverables before payment can be processed. If acceptance is pending, the invoice will sit until the COR acts.
Invoicing Best Practices
Set up WAWF/IPP immediately
Register in PIEE and configure your invoicing templates as soon as the contract is awarded. Do not wait until your first invoice is due.
Invoice on a consistent schedule
Submit invoices on the same day each month. Consistent timing helps the COR and pay office process your invoices efficiently.
Maintain a CLIN tracker
Track funded amounts, amounts invoiced, and remaining balances by CLIN. This prevents over-billing and helps you anticipate when additional funding is needed.
Get COR acceptance first
Coordinate with your COR before submitting invoices. Confirm they have received and are satisfied with deliverables. Unaccepted invoices just sit in the system.
Keep copies of everything
Maintain a complete invoice file with copies of each invoice, supporting documentation, acceptance records, and payment confirmations. You will need these for audits.
Follow up on aging invoices
If payment exceeds 30 days, contact the contracting officer. Reference the Prompt Payment Act and request a status update. Document all communications.
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