http://www.voucherexpress.co.uk/ take amex, they do charge postage but it's not a lot.

You can buy gift cards/vouchers for places which don't take amex, or simply bring forward future purchases for somewhere that does. ie. if shop say in sainsbury's normally then buy sainsbury's vouchers to use over following weeks/months and it all hits your card now.

The paypal route can be seen as cash advancing - ie. you are getting cash into your bank account at the goods purchase rate on the card rather than cash rate so it's prohibited in the t's and c's of both paypal and Amex. So it's a no go area.

If you are going to ignore this and do it anyway, at least post something on Ebay and get miles for "buying" it via the BA eStore and pay by paypal. As long as it's a buy it now and not an auction you are allowed to buy from family but don't do it as an auction. Yes it costs more in fees etc but at least you get something back ie. BA Miles