16 oz or less can ship First Class Package ($2-4) 16 oz+ can ship via Priority Mail ($8-10) And if the item fits, you can use Flat Rate Priority Envelopes ($7.33) More on reddit.com

You can just weigh/measure each item and add a few ounces/inches more to account for the packaging, don’t necessarily need to package it to weigh it. It’s better to overestimate by a bit than to underestimate and get hit with extra charges. For buyer generated bundles, shipping is automatically calculated. It gets easier after a while, you’ll be able to estimate by eyeballing! More on reddit.com

They're not letters to begin with, hence why letter postage wouldn't be appropriate. Letters can't be thicker than 1/4". You're shipping packages, which would require ground advantage postage - now you've got some options on that, and guessing here as to the weight, but guessing about 3-5oz in weight - you can go to postcalc.usps.com to enter in the package details and find out the postage, you can take it to your post office and a clerk at the counter can help you figure it out, or you can use a third party site like pirateship.com to print the ground advantage labels. An option, since it seems like these are targeting mailings in a small area, you can use uspsconnect.com and sign up for a free business account with USPS for Local Connect - that allows shipping up to five pounds each piece of locally addressed items for a flat rate of $3.95 each, often the same day if you get it to the designated entry point by the cut off time. Additionally, you'll get full tracking so you know where it was delivered and when. A second alternative would be to use USPS large envelope rate - you'll need to use something other than bubble mailers and it'd have to be under 1" in thickness, thicker than 1/4" but with no more than 1/4" variation in thickness across the entire mail piece and addressed along the edge parallel to the longest side (like an envelope is normally addressed.) Up to 3oz would be $1.87/ea for postage. More on reddit.com

International Retail Postage Price Calculator (with Canada pre-selected): https://postcalc.usps.com/Calculator/Index?country=10054 More on reddit.com