Within the US and to destinations abroad, Priority Mail is on average much faster than First Class. Looking a few packages to Canada doesn't prove a trend. The package could have been held up in customs or had some small problem with the address. And unlike most postal systems in the western world the Canadian Post, in my experience cannot deal with any variations/errors in addresses.
That being said, the last thing I shipped to Canada (by Priority Mail) left on a Monday and was in the customers hands on Wednesday.
Priority Mail also has real tracking and you can purchase insurance. If what you're shipping is inexpensive (<$25) it may not seem to matter, but remember if something gets lost and the customer files with PayPal and there's no proof of delivery, the money is coming out of YOUR account. So you will be out the article and the money.
Some countries do not reliably scan 1st Class Mail, which can open you up to the joys of being in a situation where the customer is saying they didn't get it, you're sure they did but cannot prove it. To avoid insomnia producing situations like this I use Priority Mail & Priority Express Mail exclusively.
The medium and large flat rate boxes take a 3 part, not 4 part customs form. The 4th page is your receipt that you should keep in case you have to make a claim because believe or not for foreign, the USPS not have online access to addressee information.
The 3 forms go in PS Form 2976-E (Customs Declaration Envelope)
BTW - Don't EVER use the one you fill out with a pen even if you are buying the postage at a window.
It's the small flat rate box that takes the adhesive (or taped on if you insist) one part label.
For international to most destination, the items in a flat rate box have to weigh 5-6 pounds for a medium flat rate box to make sense.