Answer:
The expression in the parenthesis refers to the first pieces Ali bought for $5, the second term +3 refers to the cost of the pack of buttons.
Basically, 2+8 is grouped in a parenthesis because both have the same cost of $5, so in this case we can use the distributive property. So, that of the expression just represents that, the same cost for the first 2 whit pieces and 8 blue pieces.
The last term +3 is not included in the parenthesis because it refer to a pack of buttons that has a different costs, it would be an error to include it inside the parenthesis.