laiz
I think that sentence may refer to being VAT (or local equivalent) deductible, since there's no specific tax line item charged.
c-alpha
When paying for Kagi, I got a formal invoice with address, an invoice number, date, amount, subtotal, and amount due. There is an option from Stripe to download both an invoice and receipt, and the receipt confirms that payment was made, and the method of payment received.
From my perspective, this seems like it would be acceptable in most "common sense" regimes as a valid invoice, with a unique reference, company name, and "bill to" details, along with itemised description of Kagi Premium with a unit price and total. Is there some specific requirement your country has? I guess Stripe might give different invoice styles based on country of residence.