I always carry ''petty cash'' for these situations. As long as the bill is less than $40 and non-business or tax-deductible expenses it's not a big deal.

Paper receipts for cash outlays that are deductible or reimbursable are a major pain in the ass. Taxi fares go on credit/debit card -- the drivers' tips can be in cash -- they like that ;) so they just pocket the money :P