There is no standard collective noun for 'envelopes', in which
case a noun suitable for the situation can function as a collective
noun; for example, a box of envelopes, a pack of
envelopes, a stack of envelopes, etc.
Time (labor), Supplies (checks and envelopes) and postage. You
can eliminate the envelopes and postage if you pay on-line. You
would still have paper costs, because you would most likely print
out a record of the payment to attach to the invoice.