This is the standard phrase that people use when they decide to buy something. You say this to the salesperson after you've discussed the item for a while. You usually use this phrase when shopping for expensive items like:
- a car
- a house
Notice that this phrase uses "I'll ___" which is short for "I will ___." One of the main ways that English speakers use "will" is to announce a decision that they've just made.
A: Oh, I love that picture.
B: Yeah? I'll send it to you.