Can a contractor invoice for G and A from the cost reimbursement travel CLIN on a FFP contract where all of the FFP line items are fully burdened?
The answer is No. While you did not mention the type of contract. I will use the time and material contract as an analogy. Per FAR 16.601(c)(3) a T&M contract has a fixed price rate, which includes direct labor costs, plus appropriate overhead (fringe benefits), plus G&A, and profit. Direct material costs are allowed plus any applicable overhead costs if the overhead costs are not included in the fixed price portion.
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In answering your question, if the contractor has included G&A in the fixed price portion (which seem logical), then the contractor cannot include G&A costs added to the travel cost. The contractor could apply an overhead cost to the travel costs if the particular overhead cost is not included in the fixed portion of the contract. The reason for not including G&A twice is double counting.
As a side note, FAR Part 31.205-46 (a)(2) mentions that there is a maximum threshold allowed for travel cost for per diem rates. Normally the maximum per diem rate allowed is that which is published in the Joint Travel Regulation.