Answer is = $17.78 total cost
Step by step
She needs 20 cards
Each box has only 15 cards so she needs 2 boxes
Each box costs $2.59 x 2 boxes = $5.18
20 cards x $0.63 stamp for each card = $12.60
Add cost of 2 boxes of cards and 20 stamps
$5.18 + $12.60 = $17.78 total cost