We've used Instacart several times on recent WDW trips. What I like about Instacart is that I can purchase items from say Publix and Costco at the same time. If you want to pick up your groceries, the driver will call you when they reach the resort. You can also instruct them to have the groceries delivered to Bell Services if you are in the park. It's very flexible. If you have it sent to Bell Services, just call them when you are back in your room & they will deliver it to you. It's very easy.
Note: If you order alcohol, you must be there to pick it up. They will need to see your ID to make sure you're at least 21.