Which shipping extension do I need?

I’m currently using the default woocommerce shipping options and shipping via Paypal.
Sometimes I get shorted on the true cost of shipping and sometimes I overcharge (and refund). I built a little cushion in my pricing to account for this but I’m shipping more and more orders and would like to be accurate. I would also like to start offering shipping to Canada (will this require a second extension). I can’t figure out which of the shipping extensions I need to get true shipping costs. I set up a trial of ShipStation today but I don’t see where that is going to help me.

I sell lots of different products in 3×3 bags but varying weights. Most are under 3 oz.
I sell one product that weighs 1/2lb.
I sell one product that weighs 1lb exactly.
Usually one of those products paired with the lighter items is where I ended up with the shipping thrown off. Or if someone orders 5 of the 3oz and pushes me out of the 1st class bracket.

I sell three variety packs that I ship in flat rate boxes so I’m pretty good there.
Customers will buy any random variation of products and counts.

I do populate the weight and size fields on the product listing.