smouse, the ruling of the new postage rules is that your letter must be maximum 5mm thick.... I would have thought that if your cardboard is thin, then your 20p pieces would fall into this category, but you need to check.... also can I say that a lot of post office counter staff have no idea about the new measurements and guess (in their favour) the new dimensions, so it is worth checking yourself. As I send a lot of books out as a secondhand bookdealer, I am used to being firm about this! Just as a PS, if you are computer literate, you can now print postage from your own computer but you need a ruler to start with to get to know the new sizes etc.