Upselling- doubling sales or hirting conversions?

Best practices anyone?

[m2c1r]

I already know the final answer is split testing :) but I was hoping for some additional advice on how to position an upsell service to go with my current products.

I sell custom software, and it would go well with an offer to review the finished product people will be creating using it- and I can offer it for an excellent price and high perceived value.

The question is, do I try and capture the upsell order on my same order page where I sell my product, or do I try and sell it in a follow up email to buyers?

On the one hand, adding it to the order page creates another decision for the buyer, and Joe Sugarman and other marketers always preach keeping it simple. On the other hand, hitting them with an email later means they have already put away the credit card and are out of the buying mood, which has to be recreated all over again. Or, is there another option?

Any thoughts or experience to share? Thanks.

[m2c1r]

I meant "hurting" conversions. Stupid lack of moring coffee...

[Sunshyn]

I don't know if there's a way you can work it with your system, but I put upsell offers on the cart page, before they get to the point where they're entering information. I put it as sort of an aside, so no action need be taken if they aren't interested. It seems to work well for us.
If it's not intrusive, I don't see how doing something like that would preclude a later mention in a regular followup email so maybe you can test both at once.