I know the obvious option is board backed envelopes, but the problem with that is the max size is 162mm x 229mm which is the same size as the greeting card envelope so doesn't fit
What i'm using at the moment is just plastic mailing bags and just sticking 'please don't bend' stickers all over them, it's working well and fairly cheap but as you can imagine it doesn't offer much protection and i'm getting like 1 in 50 coming back bent.
Any recommendation would be a big help, as said though cheaper the better even if it means me buying bulk load for 1000 (i don't want to be paying over £100).
Thanks.