Hi all - not trying in any way to dodge paying the correct postage but I've sent a couple of parcels recently which were on the boundaries between large letter and small parcel (viz, almost exactly 2.5cm thick). Who judges this and at what point is it accepted by Royal Mail that a large letter stamp/payment is correct?
To be clear

I've got a parcel here which is c2.5cm thick. If I stick it in the postbox, someone's going to assess that somewhere (where?) and potentially disagree with me, causing a problem for the recipient and a surcharge which I'll have to refund. If I take it to a post office, and they test it against their plastic measurer thing, is that then "accepted" by Royal Mail and I can stop worrying? If I buy a collection, does their acceptance of the parcel at the door mean it's in the system legally, or will someone at a sorting office somewhere do that and potentially goive me the headache later? I've not seen a collector whip out a plastic measurer yet
Suppose a simpler way of asking is, at what point in the parcel's travels does the validity of the paid postage get accepted? First time it hits an official check (Post Office counter / sorting office)? If it's safest to take any marginal parcels to a PO counter, that's what I'll do, as long as that gives me official acceptance of validity.
Ta
Rob