The order still matters. If you have $20 in your account and get hit with three debits for $6, $10, and then $30, processing them in order means the $6 and $10 debits go through, leaving you with $4 in the bank, then the $30 debit causes an overdraft and a single fee. Comparatively, processing the $30 debit first immediately causes an overdraft, and the $10 and $6 debits cause two more overdrafts for three total fees.

Either way, you're trying to charge $46 against an account with $20, but one method results in 1 fee and one method results in 3 fees. The issue of credits vs debits you mentioned can compound this issue.