Fabien,
Following on from Bruce's comment. An option I've considered (and briefly tested) is to set up a 'paypal.me' link. This would allow you to paste a link in to your Wiki page (e.g. put a pic in the page, select it and insert URL link) which when clicked will take the clickee to the 'pay' page. Seems to work ok.
Word of warning; something I missed, and evidently many others, is that you only get one choice of 'paypal.me' name per paypal account, so choose wisely if you want to use it for something else in future. Once chosen, it?cannot be changed or aliased.
Derek