I tend to use Pinflair glue as it doesn't cause the greasy marks of 'true' silicon glue - apparently Pinflair is actually silicon-free!

I do sometimes use foam pads, in varying heights, but my favourites for decoupage (not that I actually do much) are the very thin ones, some are almost flat, but you can also get 1mm ones. I find several layers of 2mm pads just too much.

I have a plastic 'Pricing by Proportion' ruler (rather than the flimsy ones from the PO) and check all my cards, with their envelopes - any which don't fit through get a sticker on the back of the bag to say that they need a large letter stamp, and any which are 'snug' get one to advise the sender to check before posting if they include anything in the card.