Do you have to pay for parking if I’m just shopping at the store

Yes

World of Disney store does validate parking. You'll get 3 hours for spending $20.

Yep

World of Disney validates your parking for 3 hours from the time you parked if you spend at least $20. If you don't find anything you want to purchase, you could buy a $20 Disney gift card to satisfy the minimum spending requirement for validation.

Yes, you definitely do. I believe I got a voucher one time because I spend a certain amount and the parking wad free on my way out