Say you are integrating some software into PayPal. Lots of people have integrated PayPal.
However no one has integrated your software into PayPal before otherwise you wouldn't be doing it.
Your software may have a weird workflow that makes PayPal difficult to integrate. So your going to have figure that out, and that's unique to your software
Your software may not have an easy way integrate 3rd party payment providers. So your going to have figure out the best place for those extension points, in reusable way for multiple payment providers. Figuring out those extension points is unique to your software.
There will be a lot of unique situations.
If however I was using common e-commerce package as opposed to some unique software, I would just install the PayPal add on. no estimation required.