No issues from me here either. Only started trading in games around 3 months ago but on each occasion I've just popped the game in a Jiffy bag and dropped it in a post box I recorded. Within 3-5 days I've got the credit
I've traded a whole bunch of stuff with Amazon and have had a reasonable service from them. For some, about 8 months ago they went through a period of losing stuff. This was particularly apparent on Vita games, even to the point where I'd sent a mixed package containing about 3 different formats and only the Vita one went missing. To be fair to Amazon though, they credited me the value each time when I emailed them about it. It also seems to have got a lot quicker recently.
EDIT: Also, always pick the highest rating you think your stuff is worth and select return if not up to value. I've only ever had 1 thing sent back and that was because it was a slightly different version of the game (original instead of platinum). I'm guessing they just match up the bar code number on them.