You could send them a PM with an ultimatum: something along the lines of
"Hi. I'm double-checking that your address listed on Paypal is correct. I've had times in the past where it isn't updated and I've mailed packages to the wrong address. I'd like to avoid this situation with our transaction. Paypal lists your address as XYZ. Please confirm that this is the correct address by (date). If you confirm your address by then, great! I'll mail your items. However, if you don't confirm your address by then, I won't ship out the items and I will issue a full refund."
I think its great that you're thinking ahead and being concerned about the correct address instead of run-around and twice-shipping payments. I know myself in the past if I think the transaction is at a point that I think its fine and I'll be away from the internet for a few days I just won't think about it until the package shows up. (I usually let the other person know though... Y'know, common courtesy and all that.)
And as DaffyDilly said, have they been on since you've contacted them? Are they flat out ignoring you? I kind of agree with DaffyDilly too, it's the buyer's responsibility to make sure their Paypal address is correct... You're being very kind and polite and considerate to do so, but don't go so far in being polite that you cause yourself a headache.
Good luck!