What ever she means, it makes no sense; excise is a tax, nothing to do with postage.

I would reply:

dear X;

I am not sure what you mean as there was no excise charged on the item.  there are 3 ways we can do this:

1 - cash on pick up

2 - PP payment including postage, and will be posted

3 - DD posted or left here till you collect

either way, I would like the item to be paid for by such&such date

yours.............

You could also offer option 4 -  delivery with extra $5 for your petrol - paid in cash on delivery, or in the bank before delivery.