Our friends at PayPal have informed us that their systems will undergo major maintenance on the 11th of July starting at 05:00 GMT and ending at 09:00 GMT.
If you buy Skype credit using PayPal at this time, there may be a delay in processing your purchase, and at times during the maintenance period it may not be possible to buy Skype credit using PayPal.
If you wish to make a purchase during the maintenance period, we recommend that you use an alternative payment option

Update: PayPal completed their maintenance at 09:00 GMT and all payments are working correctly.