Hello!

Happy to summarize (and noting that  my client uses Square for in-person transactions and is very happy with the purhcase flow: Doesnt want a different POS solution): It was very tempting to try and connect these systems in this way, beacuse they both existed and work well for what they do, but at the end of the day it isn't work the risks, uncertainties, or headaches. As a general matter, this is mostly because SquareSpace is pretty locked-down, which is totally fine if the features they offer are a good fit for your customer's website hosting needs. Remember, it looks like there was a workable solution on the SquareSpace side and the closed it because they don't want you doing this for all of the obvious and annoying reasons. So maybe we're down the wrong rabbit hole...

If SquareSquare (Website) is working well for your client outside of this. Keep it.

If Square (POS/Checkout/(basic inventory)) is working well for your client outside of this. Keep it. They appear to make it easy for 3rd parties to supply reliable integrations.... because they understand that they don't completely support your customer's use-case either!!!

If your customer is trying to build and grown an online store, they need one more product (like BigCommerce, Shopify, WooCommerce, etc). It's one more subscription, but the alternatives are effectively dead-ends.

We did the following:

• Created a store on Big Commerce
• Used the Square plugin for Big Commerce to enable synchronization between the two systems (this works well), to give the client an early preview, to get Square inventory data into Big Commerce (because you should probably handle the migration of existing products from Square to Big Commerce in a semi-manual way... Big Commerce should become the primary source of truth/managing interface for inventory (stock counts synchronize back from Square to BC very fast, so sales on the floor and in the store via a single Square store are fine) so this is an Awesome time to do a little cleanup and to create consistency in the product representations, which is usually exciting for the client), AND getting the minimal store up will give you time to...
• [Important to start early and don't tell them until it works]: Set up, wait, test, test again: the client's redirect from "https://shop.squarespacewebsitename.com" or whatever to the Big Commerce store (their docs are pretty alright). We didn't even have to jump to the expensive plan for https to work all the way through, though some historical docs and comments suggest that it used to be necessary. Thanks be to the HTTPS-everywhere people!

I pulled the Square Data into Big Commerce so I could start from a correctly formatted csv for endless rounds of downloading it from BC, making local changes via a little Python framework that I wrote to get the job done and to make it easier to a) incrementally sanitize the product data, b) respect the constraints of the BC csv upload system (The category column must have a value for each product, but prouducts in multiple categories need to have that information represented as a single delimited string, example), and uploading it again to see how the everything looked.

N.B. You have to enable synchronization to initiate the first Square => Big Commerce copy. Just be sure to disable it before you start uploading test/dev product csvs or testing your migration tool!

Cheers!

Jonathan