There is the possibility that you are, like me (but not anymore) paying as much as £0.80p extra in postage costs for what amounts to the edge of your padded envelopes, which does not even impede their progress through the larger aperture in the newly created Post office template, designed to 'make things easier for us' when it comes to posting our mail.
I have found myself being charged an extra £0.80 on a number of occasions simply because the edge of the envelope protruded over the edge of the grey area of the template used by the Post Office by about a quarter of an inch.
Many apologies if you are already doing this but please do youselves a favour if you aren't. Get yourselves down to the nearest market where you will be able to purchase either 4 or 5 rolls of brown parcel tape for £1. This tape is about 2-3 inches wide. It is no good using cellotape as it just pops off.
When you have placed your item in the envelope tape both of the long edges over. It doesn't look pretty but the whole thing takes about 2 minutes, costs about a penny per parcel and will save you at least £0.80p every time.