Most suppliers have a process already in place to purchase and pay for samples/orders from them, so I would look for their list of policies to find out what that process is. I don't recommend just going to PayPal and clicking on "Send Money" to pay for the samples. That's such a general payment method that the supplier may not be able to make the connection between your payment and your order on their website.
If all else fails, by all means, send them an email and ask them what you should do.
Regarding what Richelle mentioned, about doing a chargeback through your credit card company if you pay the supplier through PayPal and don't receive the samples -- I would only do this as a last resort, if every other possible method of recovering your payment fails.
PayPal frowns upon chargebacks, mostly because they undermine PayPal's own payment protection policies, and in many cases, if you do a chargeback through your credit card company instead of submitting a dispute through PayPal's website, PayPal will immediately and permanently close your account until and unless you repay the money.
And by the way, (attention Richelle!) PayPal DOES indeed protect off-eBay payments! If there's a dispute, the burden is on the seller/recipient to prove you received the goods you ordered.