You could weigh it and put the proper stamp on it.
The thing is, it's not just weight these days.
It's dimensions too.
This includes fatness.
If the envelope won't fit through a slot in their plastic template thing, it needs a 'large letter' stamp.
The fatness test is the usual problem with Audax entries, as a C5 envelope with a folded sheet of A4 and 2 more folded C5 envelopes inside can bloat out a bit.
Different Post Offices seem to take different views on how easily the envelope should pass through the test slot.
Some will accept a reasonable shove, and others will require nothing but gravity.
As a general rule, I put Large Letter stamps on the entry, and on both SAEs, since they can contain brevet cards, route sheets, receipts etc; and I don't want to be bothering the org with underpaid postage.