Is it legal for me as a CO to approve an invoice for snacks and meals?
In order for us to best answer your question, we would need to know the type of contract for which you are being invoiced.
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If you have a fixed price contract, your contract should contain the clause at FAR 52.232-1 titled "Payments". It instructs you to pay the Contractor, upon the submission of proper invoices or vouchers, the prices stipulated in the contract for supplies delivered and accepted or services rendered and accepted, less any deductions provided in your contract.
Otherwise, if you have a cost reimbursement or time and materials contract, your contract should contain the clause at FAR 52.216-7 titled "Allowable Cost and Payment". It will guide you on reimbursing allowable costs under a cost reimbursement contract or covering the costs of materials under a time and materials contract.