This is an old trick
Addon-Firefox Tamper Data
Google Firefox and download it
Then download this addon and install it:
When you’re done, just go to some online shops. Click “tools” in the Firefox menu bar and click on “Tamper Data”. Then “Start Tamper”
Now click on the item that you want to buy. Then a window will appear, click on “Tamper”. Another window will appear, you’ll see a label “Amount” and a text box with the amount.
Now change the amount to how much you want then click “OK”.
Another window will appear, click on Submit and then “Stop Tamper”.
Then pay it.
Note: This information is for education purpose only.