I have often used Sticky tape over stamps even on the plastic satchels just to make sure they don;t come off.  especially large or irregular items in the satchels.

Was not even aware you couldn't do that!!!

I have also submitted these satchels with stamps and sticky tape attached then Registered the parcel at Aust Post.

I have also received parcels envelopes from other sellers etc with stamps sticky tape.

Never had a problem or return