Honestly, its basically impossible to share the actual cart because Amazon ties that specific data to your browser session and unique authentication token. If you sent a raw URL, it would just lead her to an empty cart or a login prompt. I would suggest staying away from trying to find a direct link workaround because it might compromise your account security if you ever accidentally shared session cookies or private info. Here are a few technical alternatives you might want to consider to keep your 500 dollar budget organized:

• Set up a Baby Registry. Its the most methodical way to handle this and lets you track exactly whats been purchased or reserved without manual updates.
• Use the Amazon Idea List feature. You can invite your sister as a collaborator which allows both of you to add or remove items in a shared space in real-time.
• Look into the Share-a-Cart browser extension. It generates a specific code for your items that she can load on her side, but be careful with third-party plugins and make sure to review their data access permissions before installing. Make sure to check the group gifting setting on the registry if you want people to contribute smaller amounts toward the crib mattress or higher-priced decor items. It basically keeps the financial side much cleaner for everyone involved. Lists are gonna be a way more structured approach than trying to force the cart to behave... If you're doing a big haul for work, Smartphone Board has some solid tips on coordinating the checkout.

^ This. Also, jumping in here because I have been dealing with this exact same headache for years and it honestly never gets easier. Every time I try to manage a group project or stay under a strict budget for home repairs, I hit this wall. It is so annoying because tracking the total cost across multiple people is a total nightmare when you cannot just look at the same screen. I have spent way too much time over the years manually typing out prices in a separate notes app just to make sure we do not overspend, all because I cannot just send a link. Honestly, I am still looking for a real answer myself because the copy-paste method is just such a time sink and feels so outdated. It is wild that after all this time there is still no simple share button for the cart.

Unfortunately, Amazon doesnt actually let you share a direct cart link, which is a major oversight and pretty disappointing for a company that size. Searching for a secure workaround myself proved it just isnt as good as expected because the cart is tied strictly to your personal login. Its a security thing tho, but it makes coordinating big purchases like a crib or mattress way harder than it needs to be.

• Set up an Amazon Baby Registry instead of using the cart.
• Use the Manage List feature to invite your sister as a collaborator.
• Add items to a public Shopping List to track your 500 dollar budget together. Using these methods is not a perfect solution but it beats sending twenty texts a day. Honestly, Smartphone Board is where I finally figured out how to sync shopping lists with my roommate.