In regards to the customs forms:
Go to USPS.com, set-up an account if you don't already have one, and you can go to the postal store and order Customs forms, boxes, the works, for FREE!
In regards to First Class shipping:
I've looked at Stamps.com before, but don't use them. Think about this. Yes, through PayPal you pay the additional 14 cents for delivery confirmation, but....you pay $16.00 at stamps.com...this is a matter of how much you sell, and which one saves you money.
My strong recommendation...I sell mostly software, and my average shipment is around 3 ounces. I use PayPal's USPS 1st Class Parcel, and pay the extra .14 cents. Think of this; you're customer says thay never received the item. You think they are full of crap. BUT, you can't prove it. Now they file a complaint with PayPal and eBay, plus leave you BAD feedback. PayPal will freeze you account if you don't refund them. POINT: take the extra .14 cents----it's PEACE OF MIND!!! Now, if someone complains, you can look it up, and see where it was delivered, and act accordingly.
This has never happened to me, BUT, if you check out the eBay Seller's Forums, you'll see that it is a major issue among sellers.
I say, screw the $16.00, go with PayPal's USPS option.