Google Calendar connects per company and writes within seconds. Stripe takes card payments on invoices. KitSlate emails your documents under your company’s name, and replies come to your address. QuickBooks, Wave and Xero exchange invoices as files you import and export.

Each company connects its own Google account, so a studio with two companies in two Gmail accounts keeps them separate. Every save writes into that company’s chosen calendar within seconds.
Stripe handles card payments on invoices, and every document KitSlate emails goes out under your company’s name, with your company on the letterhead and replies coming back to you — not to KitSlate.
KitSlate reads and writes their formats. Export from your accounting system and drop the file in; export a CSV here to take back. You move the file, which keeps the boundary between the two systems where you can see it.
By file. Export an invoice list, a line-item export or an invoice PDF from them and import it here; export a CSV here to take back. Every movement is one you perform.
Yes. The connection belongs to the company, not to the workspace, so each connects its own account and disconnecting one leaves the others untouched.
No. The invoice carries a pay link and a QR code that open Stripe’s own payment page.
No. KitSlate sends the email from its own sending address under your company’s name, with your company on the letterhead and your address as the reply-to, so a reply comes back to you rather than to KitSlate.
The sandbox shows the calendar, the documents and the money side without any account to connect.