Once you have the product in hand to sell, make an episode about it with links to your shop and, once a month at the end of an episode you post, have an ad for your store. I would suggest with merch that you make 1-4 copies of the item you want to sell and for pricing you start at what it cost to make and ship, then tack onto it.
So if the bobble head costs $20 to make and $5 to ship, you can tack on another $25 and that's your profit that you can use to get more or to put towards whatever you wish. I usually go double the price of ship/cost so that I can replace the merchandise without it coming out of pocket. I don't really intend for profit unless I sell all of my merch in a short span of time and see a demand. As for shirts I use T-public because it's the cheapest for your customer but gives you a far better profit than, say, Society6 or Red bubble.
I hope this was helpful!
