TL;DR: Amazon doesnt let carts sync because of privacy and payment security, basically. There is no button. In my experience using Household for nearly a decade, trying to manage a $300 party budget across two separate carts is a recipe for overspending. Ive tried many ways to bypass this over the years, and the cart is always the weakest link because it doesnt show you the best deals or coupons as clearly as other tools do. Quick clarification though before I give you the full breakdown—are you both officially linked in an Amazon Household, or are you just sharing a single login on different devices? It changes which workaround actually works for the budget. Tbh, you should move everything to a Shared List right now. Lists are way better for tracking a strict budget because they show you price drops and active coupons you might miss in the cart. Plus, you can see exactly what the other person added in real-time.

Like someone mentioned, syncing is restricted. IIRC, a shared Wish List is technically better for your $300 limit.

• synced metadata Just saw this on Smartphone Board — it's a common power user snag.

Omg dont panic! Amazon carts stay totally private even in a Household. I found that out the hard way last year for my kids party when we bought double everything! Here is the deal:

• Carts never sync automatically, sadly!
• Use share amazon cart to actually see what you both have.
• Have one person do the final checkout for that $300 budget! Youre gonna crush this deadline, happy birthday to your son!

^ This. Also, honestly I'd stay away from the Wish List idea if you're on a strict $300 budget. In my experience, prices on those lists dont always update fast enough and its way too easy to lose track of what's actually in the cart vs what's just on the list. I've tried many methods over the years to keep my spending in check and the most reliable way is to actually see the items together. I'd just use amazon cart share to get everything into one spot so you can see the real total. It saves so much stress when you're trying to beat a shipping deadline... nothing worse than realizing you forgot the napkins or bought two sets of balloons at 11pm. Just have one of you do the final click so you dont get hit with multiple shipping charges or miss a bulk discount. Good luck with the party, 5 is such a fun age!