The following are a few important tips regarding the upsale function:

A product setup for upsale will only show up if the customer has not already placed the item in his basket.  If he did, then the upsale will not kick-in.  Why offer a product at a discount if the customer is already buying it at full price?

The threshold percentage of order total can be tricky.  An easy example is a $40 clock upsold against the total purchase of $100 or more.  The order total in this case is $100 therefore the threshold percentage you want is about 40%.  Miva Merchant (oddly enough) calculates the threshold percentage based on the upsale price and not, as one would expect, the actual product price.  So if you are offering a $50 clock for $40 then you use the figures above. If, on the other hand, you are offering the $40 clock for $30 then you need the threshold percentage be 30% to ensure that they spend the $100 before they get the upsale item.  Since the order total has to meet the threshold percentage requirement or be above it, keeping the threshold percentage at 40% will mean that the customer can now spend only $75 to get the upsold item. It gets trickier if you wanted to offer the $40 clock for people with a total of $30 or more.  The total is $30 and the threshold percentage will be 133%.  If you set it a little higher that will cover it too (140%).

If you want to offer the upsold item ($40 clock in this example) to anyone purchasing a particular item, let's say a $30 watch, then you will put the threshold percentage to be anything higher than 133%, then put in the price you want to offer it at, then add the product.  You will then assign the $30 watch as the required product for the upsold item to be offered and you've got it.

  If you would like to use a formula in your calculations you can use the following:  Threshold (in %) = (upsold price x 100/threshold total or price of required product).  So in the above example, the threshold will be 40 x 100/30 = 133%.

Please be aware that while the documentation claims that you can use the upsale to offer items for FREE as stated:

"You can offer your customer an upsale item, from the list of your products, free or at a reduced price."

Merchant calculates the threshold based on the offer price as spelled out in the documentation too:

"For example, you set the upsale price of the item that you are adding to be $85 and you set the threshold percentage to be 85%. The shopper will then need to have a running total of at least $100 in their basket for the item to be eligible as an upsale offer."

As you can see, the threshold percentage is calculated based on the OFFERED UPSALE price, therefore an item that you want to offer for free on a total purchase of $100 or more cannot be priced as $0 (or 0% or original price) since that means any shopping total will be qualifying (any percentage of zero will be zero which is well below anything you are selling).  If you are offering a free item with a required product that will be the ONLY qualifier regardless of the item's price.  If you have all your store items as required products, then any one of them will be enough for the shopper to receive the free item, even if the upsale item's original price is $20 and the product you are checking out with is $0.50! 

If you have a product that you want to ONLY offer as an upsale and not have it showing in the store all you need to do is not assign it to any category, this will keep it hidden but it will still show in searches and on the product list.   If you make the upsell product item that you create "Inactive" in the MIVA store (V3 or higher) it will still function for upselling purposes (as long as it is assigned as an upsell item, of course). This way, the item will not show up in searches or on the product list, but will still be available for the upsell function. (this tip came from Kevin Murphy; audiomidi.com)

Merchant V4.x allows you to offer the customer a choice of one item from a selection of upsold products.  In previous versions you can create an upsell item and call it "Special Offer" or "Free Gift" as the case may be.  Have the choices as attributes with the different items the available selections. (this tip came from Jeremy Russell; Miva corp.)

There is a bug in Merchant V2.x; a customer can buy the offered upsell item, then check his basket, click on the link to the actual product that was offered as an upsell, go to that product's page, add more, then checkout and he will get the discounted upsell price.  In such a case we suggest you put in the screen header of the upsale that only ONE "special offer" item per purchase or something to this effect.   This bug has been fixed in later versions.