I spent months on exactly this problem for a client recently. There isn't a good way to do it completely inside of Squarespace. There are too many small limitations that add up to big problems in managing the website, billing, inventory, customer interaction, storage unit locks, gate access codes, and the storage facility business itself. You have to integrate with another service through code injection on the Squarespace website you are building. We looked into other more developer friendly digital subscriptions services that integrate well with Squarespace. Those services are usually for digital products and monthly physical product subscription services. We explored how they might be customized to rent self-storage units and none of them quite work for the storage industry. Integrating actual storage facility management software with Squarespace through code injection or an embed is really the only option. We demoed every storage facility management service out there and they all have pretty big issues, especially in checkouts processes. A startup called Cubby was the only company we found that had a decent platform to work with. Here is the Squarespace website I designed and built for that self-storage facility: https://backwoods-storage.com. If you or anyone else is ever needing to hire a consultant or squarespace developer in this space I am happy to help!