Learn English – Phrase meaning whoever is buying can choose

For choose or take as one’s own, Collins Dictionary has adopt:

adopt verb

1. (law) to bring (a person) into a specific relationship, esp to take (another’s child) as one’s own child
2. to choose and follow (a plan, technique, etc)
3. to take over (an idea, etc) as if it were one’s own
4. to take on; assume ⇒ to adopt a title
5. to accept (a report, etc)

However, you might also take up the Flex plan, or take advantage of it. Certainly your sentence “I’ll think about it if I want to consider the 30-day Flex plan” might have been “I’ll consider whether I want to take up the 30-day Flex plan” (or adopt or take advantage of).

In American English, the most common phrase would be to chip in, as in I would be happy to chip in for a cake:

to give some money, esp. when several people are giving money to pay for something together

An alternative is to pitch in, though in my experience this connotes a contribution of labor rather than capital. Both chip in and pitch in can also mean to make a small (and perhaps unnecessary) comment, so don't be surprised if a request for others to chip in yields more quips than coins.

Speaking for myself, I would offer to go in on a cake casually, meaning to join or

to be involved in an activity that involves others

but which my peers and I often use in the sense of investing in or purchasing something together, like a pizza or a vacation condo.

More formally, I would offer to contribute towards a cake. That is, I don't intend to purchase the entire thing, in which case I would contribute a cake, but rather I want

to provide money or support to help another person, company, or organization to achieve its goal