Mercari sellers!!! **** bst etc

I need help!!!

This is my first time posting on Mercari, I’m selling my daughters unused or out grown clothes from a popular boutique.

I’ll be doing this a lot, since everyone loves this brand and love bst, I’ll save a few of my favorite but resell her outgrown things.

Should I buy my own shipping bags? Or when I go to the post office they have them for me to put them in? Since I’m selling multiple items at once I’m scared I’ll get confused.

I know on BST groups people sell 20 things at once, but how do they not get confused at the post office? They must bag and label themselves right? How do I do that..

I know PayPal has an option to send invoices, and print labels, but uh do I need like sticker paper or can I take it onto a bag, what size bags are good for baby clothes? Sometimes people bundle.

You may think I’m crazy, I promise this brand resells amazing! I’m on so many bst groups for it as well, and I’m just a tad intimidated.

any advice? I’m scared to start a bst post and get overwhelmed and not know what to do! Thanks!