A colleague of mine came up with a clever proof of a simple formula for all unmakeable amounts of stamps of relatively prime denominations A and B:
unmakeable amounts U(N,M) = AB - NA - MB where N and M are any positive integers such that U is positive.
Example for A = 5 cents and B = 17 cents. The largest unmakeable amount is N = M = 1 => U = 63 cents. If you choose say N = 10 and M = 2, you get U = 1 cent, which is obviously also unmakeable.
Pretty nifty that such a simple formula exists. -Carl