I feel stupid asking this 😂 mom brain

I just started a closet purge on a BST page and I'm confused on how to handle shipping. Some people are charging like $3 for initial items and $1 more until the flat rate is reached. But some are charging like $7 something. What's the difference? I have my own padded envelopes/plenty of brown boxes at home. How can I calculate shipping to be right in the cost for their invoices before I get to the post office 🤔