I got it working locally! It doesn't seem to work remotely for whatever reason, but I want to share what I did so that someone else can do this, or figure out why it isn't working remotely...
After much pondering, I figured that an easy way to accomplish this was to work in increments of the discount ($50 off) using the Discount column in Ecart. Three uses x Discount column = 150, so that is the starting value in the Visitors column (I am subtracting instead of adding).
So, I added a little bit of code right before the doctype:
$uses = $row_WAATKvisitors['coup_use'];
$used = $CedarChest->GetDiscounts();
$coupUpdater = $uses-$used;
When the customer gets to the confirm.php page, I have a hidden form element that updates the Visitors table with the value $coupUpdater when the submit button is pressed.
Theoretically, this could work with any flat-rate discount that is used "x" times.
Happy New Year!