I would say you could purchase 3 balloons.
If each balloon costs $2 and you need to pay a $3 delivery fee, then you would set up an equation:
p = $2x + $3 where p is the total price and x is the number of balloons.
Now we would substitute different numbers in for x:
p = $2(1) + $3
p = $2 + $3
p = $5
p = $2(2) + $3
p = $4 + $3
p = $7
p = $2(3) + $3
p = $6 + $3
p = $9
Now, knowing that we have $9 to spend, the max amount of balloons we can buy is 3. :)