I worked on a web store once that depended on a couple of 3rd party web services and set up subscription billing. The project turned out to be like an iceberg,

I worked on a web store once that depended on a couple of 3rd party web services and set up subscription billing. The project turned out to be like an iceberg, where 10% of the work went into features, and 90% of the work went into heading off all of the various possible failure modes. When the system failed, people's credit cards got billed the wrong amount and either the store didn't get paid or the customer got overcharged. So I can relate to all of the effort they put into fail-safety.