Coupon applies to multiple line items with MarketPress plugin

Using MarketPress, if I create a coupon code with a specific dollar amount (say $40), that dollar amount applies to every line item in the shopping cart.

For instance, if I have two different t-shirts that cost $45 each, and apply a $40 coupon, the customer will be billed a total of $10 instead of $50.

It appears the coupon is applying to each line item, not to the subtotal.

Is there a fix for this?