Make sure your seller is willing to ship to an address that is not on your PP account before you pay. In fact, this problem should have been discussed before the Offer.
A seller who ships to a non-PP address gives up his Seller Protection.
You cannot add a foreign address to your Canadian PP account.
The simplest thing is to have it shipped to you in Canada, pack it , haul it to California, use it there, repack it and bring it back.
You will not pay customs to bring the item into the USA, since it is personal property. Like your toothbrush or iPhone5. Same when you bring it back into Canada. If it is something esoteric, you should bring your eBay and CBSA paperwork.
I would also suggest that you have the item shipped by UPS EXPRESS. This is quick, reliable, fully tracked and while it seems expensive, it includes the customs brokerage fees that are such a shock when a UPS Standard item arrives on the doorstep.