The thickness of the 3D foam pads is not much of an issue when you wnat to hand cards with decoupage over. For those projects I love the 10x10mm large foam pads (unless the design is too fiddly and requires a smaller size). I use the ones from Dovecraft, but I think others make similar ones (e.g. Crafter’s Companion).
But if you want to post them, the postage can take a hike for thicker envelopes. I am then using 5x5mm foam pads with 1mm thickness or silicone glue, so that the end result is not getting to bulky. I usually stock up at a local craft shop or at the Glitter Pot. Joanna Sheen is also always worth a look, as she doesn’t charge for p&p and has competitive prices.
For sticking on gems, etc. I am using Hi-Tack Very Sticky Glue, an all purpose glue. I have not only seen it in craft shops, but also in haberdashery departments.