Variation of knapsack problem I have a menu of n items, with each item having a value. Given the total amount spent, I have to figure out all the possible combinations of items purchased. Example, I have three menu items:

item 1: $4

item 2: $5

item 3: $8 

with the total purchase price = $14

There is only one solution in the case, 1 purchase of item 1 and 2 purchases of item 2. 

How do I go about solving this?
This is called the subset sum problem.  There's lots written about the problem; go read about it.

How you could have figured this out on your own: you already know it is a variant of the knapsack problem, so if you had read the Wikipedia page on the knapsack problem, you would have encountered a description of the subset sum problem in the section of that page on variations of the knapsack problem.  In the future, I encourage you to do  research before posting, to make sure your question isn't already answered in the obvious places (e.g., Wikipedia, textbooks).