If you already have a custom chrome extension running on your browser and you want somethig new, not complex enough to require a whole new extension running alongside the previous one, this is the best option. Your previous extension updated at a reduced price to implement whatever needed. Also to fix bugs that appeared in your custom extension.

Requirements analysis: The source code is provided and enough information to understand the owrking flow of the entension. All the specifications of the implementations/bug.

Set the course of action: After a thorough review, a course of action os presented, with an estimation of the duration and initial price.

Final product and test: the final product if delivered, and everything that fails regarding the modifications done is patched for free.

Payday: After a 20 days period after the final deliverable with no errors, the final pay is made.