I've always used scotch tape to tape the cards together. I always fold over one end of the pc of tape to make a little tab to get ahold of the tape to remove it easily without damaging the sleeve. I've never had any complaints using this method.
I also use greeting cards for stiffeners in the envelopes trades are sent in. The ATCs are taped to the inside of the greeting card, extras are put inside the greeting card and a note written on the inside of the greeting card. If you trade with me, you are likely to get a halloween card at christmas or a christmas card in july or a get well card when you feel absolutely fine! I don't pay attention to what the greeting card is for, I pay attention to whether the trade will fit comfortably in the greeting card.
I've never been charged more than a first class stamp for a greeting card with up to 4 ATCs and a few extras in it. Most of my cards are flat, without embellishments to add extra thickness or weight to an envelope. Those that do have embellishments, I pay the extra for.
Greeting cards can be had for a song at the thrift store. I buy them ten at a time for 50 cents. I've also found them at yard sales in the free box on many occasions!
Your tutorial on packing and mailing is quite helpful, OY!