Not had much luck with Customer Support on either end – both Royal Mail and Squarespace expect me to 'contact my IT department' to whitelist IP addresses on my servers. No idea what I'm supposed to do with that info since both of these platforms are supposed to be managing this on my behalf so I don't need an IT department. Not sure who owns the integration but it's not great.
Anyway, current workaround I've employed is to install the Click & Drop Desktop App – this sets a folder on your desktop that you can download your Squarespace orders to and it will bulk upload them into Click & Drop. There's an initial manual upload of the orders csv file which needs mapping to the required fields in Click & Drop. After that it looks like I'll need to download the orders csv file every day with the latest 'pending' orders. You can start this process from the Click & Drop integration page and it has all the relevant instructions to get set up.
Not perfect but better than the manual inputting of data I have had to do for the past 4-5 days. Hopefully it will be possible to escalate the problem and it'll get sorted 😞