You can find out pretty much all you need to know about mailing at the PO's web site (
www.usps.com). Click on International and you can find out what it'll cost ahead of time by filling in the information on the box size, weight and value. You will need the postal code for ETI as they charge a different rate, depending on how far the package has to travel.
Just be prepared for some sticker shock on mailing to Canada. It's not cheap, compared to mailing the same package to the USA.
You can fill out the customs form online as well as paying for it online. You can also schedule a pickup from your carrier (for free if it's part of your regular delivery, as opposed to a certain time to come by to pick it up, which will cost extra). When you fill out that form, they state at least one package has to be priority or express for a pickup, however if you fill it out as saying you have one of those and you really don't (as in parcel), they still pick it up. Heck, they want your business!
I do 99.99% of my shipping online. And I ship a lot, between my business and personal stuff that I'm selling.
If you don't want the carrier to pick it up (such as you're working and won't be home), you can still pay for it online. Just leave it on the counter at your local office and state it was paid for online. that way you won't have to wait in line to pay for it.
If you don't have a postage scale, you can still get the weight of the box by using an accurate bathroom scale.