In App Store Connect, click to Agreements, Tax and Banking > Contact Us (at the bottom) > select Reports and Payments > Tax and Banking Setup > Contact Finance > Invoicing Apple for Apps. In the Invoice Upload field, select your invoice file in PDF format. Apple Distribution Interntional's VAT ID is IE9700053D.

You can generate a campaign link. More info at App Store Connect Help – Measure campaign performance - https://help.apple.com/app-store-connect/#/itcfa7936330.

It looks like the StoreKit 2 API is Swift-only.

Let's not forget that in many countries it is mandatory to register for VAT only after a certain turnover (in that country). It's often not like in the EU where you register for VAT from the first sale to a non-VAT foreign person (I'm not sure all EEA countries have it that way). (This is not legal advice.)