Cross-selling and upselling get used interchangeably, but they are different moves. Upselling makes an item bigger or more expensive. Cross-selling adds something that belongs with it.
On a digital menu, cross-selling is the easier of the two to get right, because the system knows exactly what is already in the order.
The temptation is to attach your highest-margin items to everything. Guests see through it immediately. The suggestion has to make sense on the plate first — margin is how you choose between two suggestions that both fit, not the reason for the suggestion.
Dish-and-drink pairings are the most natural, because guests already expect a recommendation there and often want one. Shared sides for tables of three or more work because the decision is collective and nobody wants to be the one who ordered wrong. Desserts convert far better when suggested during the main course than after plates are cleared — by which point the guest has mentally finished the meal.
Suggesting things that duplicate what is already ordered. Suggesting a heavy side with a heavy main. Offering the same three items to everyone regardless of order. Each of these teaches guests to ignore suggestions entirely, which costs you the ones that would have worked.
A named combination — a main, a side and a drink presented as one thing with one price — converts better than three separate suggestions, because it is one decision instead of three. It also lets you build the margin in at the combination level rather than item by item.
Every restaurant has pairings it is certain about that guests decline, and combinations nobody predicted that convert. A digital menu tells you which is which within a few weeks. That is the part paper cannot do at all.