So you would recommend implementing authentication and handling credit cards from scratch? The reason I'm going with this approach is because although I do use a monolith and a single (hosted) DB for my business logic my understanding was that implementing high-impact, high-complexity (non competitive advantage) systems like authentication and credit card handling are best 'outsourced'.

This outsourcing of high-impact, high-complexity does come with the disadvantage of now having to deal with external services. As far as I can tell, the atomicity/transactional problem with these external services generalises to microservices - hence the microservices tag.