Paypal receivables account has no overdraft protection and not overdraft allowance. If a withdrawl over the value of the account comes in, it is denied.
Connect that account to paypal.
Next, in the Paypal receivables account, setup an external transfer from the receivables account to the main account. You will have to validate the ownership of the second account usually by posting two small transactions that were auto posted to the account as part of the validation process. This takes a couple days. Once that is done, you can transfer money from receivables account to main account but not the other way.
Transfer money out of paypal to the receivables account, transfer money from receivables to main. Use a credit card to spend money with paypal.
You can set up two way transfers if you need to put cash into that account for transactions and you still get a similar level or separation, I just prefer to use a CC instead because they'll help you out if you want to dispute or reverse a transaction and paypal may not.