We made CrossSelling table, linked to Item card. On this table, you can choose Item or Nonstock Item and put the CrossSelling Item No. Also, you can make CrossSelling combination per item variants and/or use CrossSelling Item variants. Same as substitutions, we can make Condition (new table) and Interchangeable.
When you make sales order, for each line you can see number of cross selling items.
If you go to cross selling form, you can see all items for cross selling. Here, you can choose all of them or only one or few of them. Also, you can see their Inventory and Quantity Avail. on Shpt. Date based on Shpt. Date from order. You can see specific condition at each line if it exist. When you choose and come back to sales oreder, choosed cross selling items ar added in new order lines.
Briefly, this is how it works my customization.