You can just send in a standard envelope to the Freepost address. Word of caution, go to a post office to send them off and get a free proof of postage. I sent off 2 envelopes with mobile phones in, didn't think twice about getting PofP and when chasing Avios for the points months later, they claim to have never received the items. Strange that 2 separate envelopes posted apart can go missing. They said to send in the PofP and they would honour the points, hindsight eh!