Postage/after auction question about shipping

[ButterflyEffects]

Good afternoon,
A simple question.
I will have over 6000 items for sale very soon. As you can appreciate there may be multiple purchases. I wanted to be able to calculate the shipping for the required items and then send the customer and invoice for the goods and shipping.
I have trawled the forums but can't find a definitive answer.
(a) if I use shipping option 01 (unknown) on the Bulk Uploader will this mean I have to send an invoice before they can pay? If so then that is the answer to my problem.
(b) if (a) is not correct then, aside from Notices, how can I let people know that postage is extra and based on the weight of the item(s)?
I hope this is clear. I'd like (a) to be true so I am able to fully manage the postage issue before the customer pays.
Many thanks
Patrick

[raindropsies]

If you use unknown that will not stop people paying before you send an invoice. Also this will effect sales as people want to know what the total cost is upfront. That is also a DSR requirement

(b) just list with postage price and state that you do apply a discount for P&P on multi-buys (or words to that effect)

[ButterflyEffects]

Ahh, now i understand the 'unknown' implications.
I am using the Bulk Uploader spreadsheet, so I don't think I can put any additional text in there - or perhaps Bulk Edit would do this?
I will perhaps see what happens when I use a standard postage rate and somehow let people know about the multiple discounts.
Cheers