Stardew Valley Map ($25)
Stardew Sticker Pack #1 ($10)
Joja Cola t-shirt ($17)
Stardrop Harvest t-shirt ($17)
Cave Dwellers t-shirt ($17)
These are all official merchandise, so you can be sure that ConcernedApe gets a cut of the income.
In addition to the cost there is also a shipping cost, which is $5.5 for international t-shirt shipping, and $11.75 for international map shipping.
Redbubble also sells a few Stardew Valley merchandise items, but this is not official merchandise.
If you are looking to buy the OST of the game, it can be bought digitally at the official web store.