Seems like you're having a problem with both english and maths.
Let me translate and break it down for you.
"the item price was $399.99 and you did put down the $40.00 deposit" - $359 ($399 - $40)
In fairness, this bit is what causes your confusion, but it's just a "For Your Information" nugget.
What follows is the important bit, and you need to forget whats just been said as we start right back at the total cost of $399
So, now the misleading bit...
"in total" - $399
"plus shipping" - $18.15 ($399 + $18.15 = $418.14)
"plus taxes" - $30 ($418.14 + $30 = $448.14)
"add the credit" - -$33.75 ($448.14 - $33.75 = $414.39) -bad terminology, we're actually taking off the NRD, "adding credit" confuses things
It seems you're discounting your NRD twice in your calculations
Hope that helps.