Well, then use your credit card, it should not be an issue because you most likely still need a credit card to verify your namecheap account. Is it possible that you do not have sufficient balance in your namecheap account ?
Namecheap and GoDaddy are not as cheap as they pretend and both have twisted PayPal schemes. Even though I have not faced funding problems, but they might arise if you are not a paypal verified member.
I have not paid namecheap these days. But that was not asking for anything exept paypal about some months ago.
You could always try the Paypal Virtual Debit Mastercard, works like a charm for me and keeps places from charging paypal transaction fees, since they just think it's a cc.