Paypal standard doesn't work with coupon

i've opencart 1.5 i seen that paypal have problems with coupons

i set a discount (10%) and the total amout is correct, but if i click confirm order cant pay with paypal ( uncorrect amount )
so i opened this: catalog/view/theme/skin/template/payment/pp_standard.tpl and added this at last line
it show: "-0.05" so the problem is that paypal stardard didn't see the total but just the coupon value , of course paypal show the error invalid amount.. the custom should have 0.05€ for the sale?

How i can solve it?

no one seen this bug?