The best thing to do is read the product disclosure statement that comes with your visa debit card. I've had numerous such cards over the years, and they've all been the same, but you never know, things may have changed, and I'd hate to give you bad advice.
On top of my own [relatively extensive] experience with credit/debit cards, I've integrated countless payment gateways for websites that process cards, and I'm also a certified Paypal developer (which means my name is on a webpage at Paypal.com and I get a lot of spam as a result)
Regardless, usual disclaimers apply.
1. In my experience, if you have a Visa or Mastercard "debit" card, you are afforded all the same purchase and fraud protection that comes with a Visa or Mastercard "credit" card. So, when I say "credit card", I'm referring to both.
2. When you use such a card with an online purchase, one where you don't sign, it's called a "Card Not Present" transaction. If there is a dispute for such a transaction (e.g. fraud or goods not received), the burden is entirely on the merchant to prove you knowingly requested and received the goods.
3. Also, with both Visa debit and Visa credit cards, with transactions I've disputed, the disputed amount is always credited to my account as soon as the dispute begins. If the dispute would have been found in my opponent's favor (never has), the money would have been re-withdrawn at the close of the dispute.
4. Even if you use this credit as a Paypal funding source, all of the above still apply.
Most websites don't actually ever see your credit card. When you press submit after entering it, their web server sends it to their bank, their bank comes back with a Yay or Nay, and a transaction number. iTunes, Amazon, Sony and a few others do store your card. For them to do this and maintain their contracts with Visa/MC, they have to be audited and certified. In light of Sony, this actually doesn't mean much. But unless BBTS lets you STORE your credit card, instead of asking for it every time you make a payment, no one who works for BBTS will likely be able to see your credit card number.
However, let's say BBTS, their processor, or the douchebag that installed a keystroke recorder on your PC, do in fact get your card number and use it maliciously, just see opening points #2 and #3.
So, does this mean that before you go to the merchant, you login to Paypal and choose "Add Funds", then wait 3-5 days until Paypal says the money is there, and then you go to the merchant, say you want to pay with Paypal and use your now existing balance?
OR, do you just go to the merchant, choose Paypal, pay by eCheque (aka Bank Trasfer), and let Paypal then suck the exact amount out of your bank account and give it to the merchant?
If it's the first one, you will probably get some additional purchase protection from your bank. Otherwise, you will only have Paypal dispute protection, which I've had bad experiences with, and am not fond of. If your choice is these two, I'd definitely do the former.
I guess it all depends on how much you're talking about (which I don't need to know). It certainly can't hurt. However, if you have created a dedicated transactional bank account for toy purchasing, add that Visa debit card as a credit card to your Paypal account and fund all transactions with that. The separate bank account protections your savings, you'll now have instant purchase ability as well as the maximum possible purchase protection.
Was that all confusing enough?
Remember, the speed of Paypal depends on how you fund the transaction as well as whether you have a credit card attached to the account.