Hi Julie! We worked with you via chat yesterday to set something up on your website, but for anyone else browsing the forum for this topic here's a general idea of how to offer upsells with E-junkie:
We aren't able to adjust a transaction once the credit card/buyer information has been entered, so any additional promotional items need to be added before checkout is started. This can often be done via the design of your sales pages. For example:
The first button for your product is actually a link to an upsell page that contains the actual button for that product alongside a special bundle or alternate item that contains both the first product and the additional upsell items. The buyer can go on to complete their purchase with the option of their choice.
Other possible ways to cross promote items after the order are to include discount codes or offers for related items on your post-sale thank you page, which can be set on a per-item basis. It's even possible to add the button or purchase link for a related product directly in your thank you emails/pages to allow someone to make another purchase right away.