Complete the sentence with the recommendation form using should or ought to, not a suggestion with could: The accountant says this is a recommendation, not a requirement: if the invoice total differs from the quotation, you ___ both documents before paying. (check)

Explication

Should check and ought to check both express the stated recommendation after a real condition, which is the form the sentence asks for. Could check would offer only a suggestion of possibility, which the instruction excludes, and must check or need to check would turn the advice into a requirement the premise rules out.