Aufladen.de, a reseller of recharge vouchers, recently started accepting PayPal which finally allows foreigners to buy from them. Before they accepted credit cards but only if issued by a German bank.
PayPal payments are subject to a an additional fee of € 0,50 on top of the face value of the topup voucher.
Recharge codes will be displayed and also sent by email immediately after purchase which you need to enter according to your provider's instructions. Lidl and Fonic customers can enter recharge codes online on the customer portal by the way.
The service is run by a Dutch company which offers the same thing for the Netherlands at
http://www.opwaarderen.nl/ which allowed me to recharge my Dutch T-Mobile SIM card using PayPal. Before they only accepted Dutch credit cards for payments on the Dutch platform.