We used Dizzy Dolphin and were very happy with them.
Basically, you fill out an online form listing all of the items you want and desired quantity. If you want/need a specific brand/size, etc., you can make note of that, and we found that they followed those requests. We were traveling with someone with celiac, so many of the items had to be gluten free. They were excellent about ensuring the requested GF items were actually gluten free, even when the brand we requested wasn't available.
I believe there is a minimum order of $50, plus a delivery fee of $14 (unless your order is over $250, then the $14 fee is waived). It seems like they just had standardized prices for the items, not necessarily reflecting what the actual paid amounts were. We found those prices to be pretty fair though.
They give you the ability to request a quote before you place your order, so you know what you're getting into. You can then add/subtract items aaftwr seeing your quote. We considered the fee/minor price markup to be worth what it would have cost us to get the groceries ourselves, and other delivery companies seemed like they would have been more for our same order.
Overall, we were happy with their communication and service and would definitely use them again.