How do I sell upgrades?

[goonky100]

Hi there,



I sell books online. I would like to allow an upgrade purchase. So for example, someone buys book 1, then decides to come back later and buy another book. I'd like to offer an upgrade price.



How do I do that?



How do I know what they already purchased?



When they come back to do the upgrade, do they enter a transaction ID?



Any advice / help appreciated.

[E-junkie_Guru]

You might adapt our method for selling Gift Coupons:

http://www.e-junkie.com/ej/tips.gift-coupons.htm



Instead of a flat amount, you could set the discount codes up for a percentage discount, and instead of selling the codes themselves, you could just set up each book product to also Send Stored Codes. For every book someone buys, they would get a single-use discount code they could use on their next purchase.