don't use paypal for paying for seeds. most seedbanks don't use paypal anyways... most everyone will recommend that you just send cash, even though you risk the chance of loosing your money. the risk of loosing $100 cash is better than leaving an easy paper trail. just my 2 cents...
if by 'legal' you mean a medical marijuana patient, then you are still not in the clear. remember customs and the USPS are run by the FEDERAL government, which means they consider any marijuana product 100% illegal regardless of the state you live in or your MMJ status. so when it comes to ordering seeds online, your MMJ status is 100% meaningless. but if by legal you mean you live in the netherlands...then i don't know why you're asking all this. just buy the damn things.
Money order. If you're sending cash, get a prepaid envelope that gets tracked online by the post office. Xpress Post if you're in Canada.