There are no FEDERAL laws restricting ammo purchases, especially since you're a US citizen. I don't think that there are any PA laws either. Your potential problem areas would be transporting the ammo through NY (unlikely to be a problem) and getting the ammo through Canadian customs.
You're in a better position to research Canadian import requirements than I am.
P.S. I'd do some shopping in advance to find a source of 300 Savage before I left
Thanks; I did source the ammo that I did end up purchasing in PA. I had absolutely no issue on returning to Canada; I showed them my PAL and I was good to go. They didn't even pull me in.
Another trip they did ask to see the 2 magazines I picked up for my Tokarev TT-33; they were only wanting to know if it was under the 10-round limit Canada puts on handgun magazines. Again, they took a look, looked at my PAL, and sent me on my way.