Time to Start Selling

[TheArtBard]

LEVELLING UP
I'd love to hear what y'all have been doing to sell various products.
Who's developing your goods?
Are you selling via a third party (Fine Art America, Printify, something else?)
Do you prefer: Developing your inventory in bulk, storing it, and shipping it as the orders come? Or letting a 3rd party do the order fulfillment, and just take your profits from the mark up?
How do you add the personalized touch to orders if they're being fulfilled by a third party? (Thank you cards, custom boxes).
Are you using a certain formula to figure out how much to sell of one type of product, and why?
Please, if you could, walk me through your mental considerations and how you're coming to conclusions. It would be SUUUUPER helpful~
Definitely interested in what @Lee-White, @will-terry-art, and @Jake-Parker are doing for this~

I'm setting up a shop. My main products are likely to be:

Greeting and Holiday Cards
Puzzles
Prints
Journals/Notebooks
Mugs/Cups
Pillow Cases

And eventually down the road:

Coloring Books
Sticker Books

Thanks in advance! Hope y'all are well and keeping busy~
Shani