Im super satisfied with how I managed my last big bash using a shared account. It worked great.

• Are you guys on the same Prime?
• Is mobile access important tho? Just curious what setup you're using.

To add to the point above, I actually disagree slightly on the shared account idea. While it works for some, I’ve found that sharing passwords or even using Prime Household can get really messy with your private order history and personal recommendations getting all tangled up. I’m a bit of a stickler for keeping my account separate but I’m super satisfied with using the Idea List feature instead. It’s probably the closest you’ll get to a collaborate button. The technical reason you can't find a share button is that the shopping cart is tied to your specific browser session and encryption for security. Honestly, it makes sense. Here is how I usually handle these big group buys:

• Create a new List and select Idea List.
• Hit the Invite button and make sure you select the option that allows others to add and remove items.
• Have your sister add everything there so you can see the running list in real time. Just saw a post on Smartphone Board about this—apparently, it's a common snag for power users who want more collaborative tools in the app. Once you’re both happy with the list, you just click Add all to cart on your end. That’s when you’ll see the final total with your specific taxes and shipping. It works well and I’ve had no complaints doing it this way for my own events. If you get stuck on the invite permissions, just let me know.

Ugh, just catching up on this. Honestly, it is so disappointing that Amazon still hasn't rolled out a native collaborative cart feature for their main retail site. They have the backend tech for it in the Amazon Business and Fresh sectors, but for us regular users, it is just not as good as it should be. I have spent way too much time testing browser extensions that claim to do this, but they usually have major sync lag or just straight up fail when you try to checkout. It is a total headache tbh. Since you are trying to stay under that $300 limit, are you guys mostly shopping via the mobile app or are you both on desktops? It makes a huge difference because the way the cart session tokens sync is totally different between the two platforms. Also, does that $300 budget need to account for the weirdly high service fees for the bulk snacks if they come from the grocery side of the site? If you tell me which platform you are sticking to, I can probably give you a better technical workaround that wont result in you guys accidentally double-ordering or missing out on the tax calculations until the very last second.