I highly reccommend that you download a copy of PayPal's "Website Payments Standard Integration Manual"!
There are dangers involved unless you install some of their security features, when you make your PayPal buttons!
One very scary danger is that someone could "copy" your product page html coding, then change the price in the coding, publish it into their website and proceed to purchase your product at a reduced price!
Better get this (pdf) document and read all about verifying payments against the item ordered AND encrypting the PayPal button coding!
Tons of other things that you might want to know about in that doc. too!