Hey andrew,
My suggestion is to add more payment providers i have one in mind for you
srpoints.com
they pretty good i use them on my text based php game it allows many many payment options and you could run it alongs side paypal options you have this way anyone and i mean anyone would be able to donate to your server by phone cc google amazon the list goes on and on they have over 50 providers incorparated into there offerwall

its easy to make a call back file in php infact i could make one for you if you like maybe you could use it to make donation points for player on forum then allow them to use those points for features ingame like you do with the cart except have them put in username and then deduct points fromthere forum account and save those to a sql db for you to go through and give out its just a suggestion but could bring you more money to server as i can bet allready other are limited to donate due to only paypal accepted

srpoints also pay out to paypal and is super easy to set up(i could help you there) i dont know what forum you using and if its able to be adjusted like phpbb was on my game forum i had ultimate points set up and used only for donations and offerwall just added points to there account as they donated

the best thing is it allows users to do free offers like surveys faceook app registers ect ect it has paid offers or straight up direct donation option

if you would like to talk about it i can go on vent talk to you skype or just forum pm me ill answer

what dose everyone else think of this