Sum of all items including tax is

$$ Total\ cost\ of\ 5\ items=\$ 18.55 $$

Let S be cost of items excluding tax:

$$ S(1.06)=\$18.55 $$

$$ S=\frac{\$18.55}{1.06}=\$17.50 $$

$$ Cost\ of\ 4\ items=6+2+4+3=\$15 $$

$$ \therefore Bag\ of\ balloons=\$17.50-\$15=\$2.50 $$