subtotal and negative balances

[istdesigns.com]

Hello all,
I'm fighting a problem right now where we send out some parts that are inexpensive, and we use a "free shipping" coupon. The parts cost is less than the Fedex shipping charge and corresponding coupon amount. We just put the items in an envelope and mail 'em because I can't see charging our customers $8 shipping on a $5 part.

First oddity: The coupon drives the subtotal negative, because the subtotal reports an amount net of the part and free shipping. The report display order is subtotal, shipping, coupon, total. I know you can mess up the math if the order is wrong. Anyway, the net result is a declined charge via the gateway because the subtotal is negative.

Second oddity: Half the time this reports the right value in the subtotal! However, if you hit refresh it will bomb and give a negative subtotal (it is NOT subtracting the free shipping twice, though, it just changes the order in which it calculates. It displays in the same order every time, just the amounts can change...odd!!).


Sort order in the admin order total module is:

1. Subtotal
2. Shipping
3. Discount
4. Order Total

The other options in the order total module are off (tax, low order fee, gift coupon, etc).

I've tried everything and can't figure this out. I've searched the forum also, and all solutions were "make sure the sort order is the default" which it is, I've even tried the sort order number values of 800 for total, etc.

Any ideas? Seems to me the ot_coupon.php or the ot_subtotal.php is coded wrong somewhere and needs a minor tweak. Or something is corrupt, but we have not had a problem before trying this "free shipping" coupon on low-cost items.

Actually, the ot_subtotal.php module shouldn't even be looking at discount coupons. That is the problem! They should be addressed in the total only, not the subtotal.

Thanks for any and all help!!!