In the past I have used International Postage Prepaid Envelopes, the C5 one allowed up to 125grams and 20mm thickness. I think it actually worked out more economical than using my own mailers and buying stamps seperately. But I bought them as 10 packs rather than singly.

Not sure if they are still available, the guide I linked to only shows DL and C4 sizes. Still for me I prefer NOT to use pretty stamps for a couple of reasons.