What are you trying to do exactly? You don't have to use the subscription tab or anything. You can link to plans directly or even use their basket URL to directly link to a basket with that plan selected. Both URLs can be found at the top while editing your plans in CBSubs > Plans.

There is no API documentation beyond integrations and gateways, which you can find below.

www.joomlapolis.com/downloads/documents

I guess you could have a UI for them to select various things that then sets a price value in a CB field. Then use a negative CBSubs Promotion substituting in that field to increase the price of a plan. They'd then just purchase that plan and its price should be based off your field. Probably the easiest way to go about it without requiring a large amount of coding.

You could probably use CB Auto Actions to bridge your plan selection UI and CBSubs together by having your plan select form submit to an auto action (leave Triggers as None and use the URL for the auto action under the Global tab as the form URL). Your auto action would be responsible for setting the value of the field (you'll need to use a Code action and some PHP for that). Then your auto action could redirect to the basket URL of your plan. So the user would experience the following: select plans > submit > at basket to pay.

You can purchase multiple plans at once. Simply set them as non-exclusive. Have a plan for each option and just let them build the subscription they want from those options.