However, I'm from The Netherlands and I don't want to pay for a subscription for international phone/text messages. As the whole purpose of shopping on Appsumo is to pay once and never have to pay subscription fees later on.

Do you have, or do you plan on integrating with other message services such as Slack, Whatsapp, Facebook Messenger, iMessage, etc. etc. as alternative to paid text messages?

In that case, you'll have a new client :)