Combine similar items with different counts for ordering

As a user, I want to summarize similar items so I can order the correct quantity in whatever size is available on order day.

I want to count each SKU separately, then choose which SKU I want to order that particular week. Based on the count per SKU, my total order history of all related SKUs and the count of my selected SKU, I want to know how many to order of that specific SKU that particular week.

Case study:

I order Carprofen chewable tablets. The availability is spotty for certain size bottles, and I prefer to order the biggest bottle. If the biggest bottle is not available, I need to order an equivalent amount in smaller bottles.