I've looked at other topics related to this question, but mine is quite different from others so I'm not sure about the answer.
Let's say I am building a website that will provide SAAS for users. In my source code, I am using parts of another source code released under GPL v3. Not an external library, but code completely integrated into mine (around 5-10% from all of the code). Users will interact with the website on a subscription basis. I am not selling the code, just providing a service. In this case, I've understood that I'm not obliged to provide my source code.
But if what the following case: I provide another subscription, where the users will be able to download 1 file and placed it on their website. This file will be 100% my code and will appear let's say as a widget and will allow the user and other visitors on their website to use the service I provide, directly from within their site. But at this point, this will not be a "service" but a kind of delivery.
In this scenario must I provide my source code? I think the answer should be NO because I'm still providing a service. If the user' subscription ends, he will not be able to do anything with that file anyway.
Just to be clear, the open-source code I'm thinking to use will be no more than 5% of the end code. It's some class methods related to processing post data from forms, that implements a bit higher level of security and cleaning of the post data.