Hello @GiantSquid,
Welcome to the PayPal Community!! This is a great question. There's not a way to add funds from a MasterCard to your PayPal account, but I was able to find an article to help guide you through adding funds onto your PayPal account. It does however involve adding and confirming a bank account first. Here are some resources you may find useful:
Do I have to add funds to my PayPal account to use PayPal?
How can I add funds to my PayPal account from a confirmed bank account?
Best wishes!
- Jon
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