My utility company reminds me every month not to send in stamps for payment. (?)
I can’t imagine that every utility company is as forward- thinking as Cincinnati Gas & Electric, though…
Soooo, I’d put an old .33 stamp in there* with a note to "Keep the change." Worse comes to worse, they'll have to pay another .34 to tell you that they can’t accept a stamp as payment.
*And we surely have old .33 stamps around since nobody seems to have the .01 stamp here in Cincinnati. I’m suspecting it wasn’t even printed. This is yet another conspiracy against the consumer, but that is another thread.