If you want to write it on an envelope which must be kept flat, I think the verb must be 'torcer', which actually means 'to twist', but the other verbs to do with bending, like ''inclinar' and 'doblar' are more to do with bends in rivers or when a person bends down. Having said all that, my missis, whose Spanish is less rusty than mine says she'd write 'No se puede doblar este sobre' or just ' No doblar este sobre'
But to be safe (from a postman who knows) use either a board backed envelope or use a bit of card inside an ordinary envelope. It's not cos we can't read, but in the course of transit up to delivery it can go through so much - machines, stuck in the bottom of a container with other stuff on top, etc.