The certifying officer assumes ultimate responsibility under 31 U.S.C. 3528 for the validity of the claim; however:
(a) The traveler must ensure all travel expenses are prudent and necessary and submit the expenses in the form of a proper claim;
(b) The authorizing/approving official shall review the completed claim to ensure that the claim is properly prepared in accordance with regulations and agency procedures prior to authorizing it for payment.
You should consider limiting the levels of approval to the lowest level of management.