If you do donate, it might take a while for it to process, it usually doesn't take long, though. If it does take longer than expected, you can make a support ticket by going to the UCP and clicking the 'Support Tickets' page. From there, press the 'Create a Ticket' button and explain what happened, after a while they should respond and ask you questions (such as your paypal email, etc.) and they'll be able to help you.
I've donated quite a few times and I've never had issues.
Once you've donated, be sure to type /donation in game to receive your benefits and /donationvehicle to receive your donator vehicle. Be sure to do this on the character you donated for.