Automatic Discount on Variation

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Hi there

Just wondering if anyone has found a solution for the following problem: 

Our products come in 3 sizes (small, standard, large) and in 2 versions (left, right).  In Shopify each combination of size + version creates a variation for that product.  So, I have a product page with 6 variations.  I want to apply a  discount whenever a customer buys a pair i.e. a left and a right version.  The discount however, should only apply if the same size is bought i.e. a small left and a small right.  I don't want the discount applied across sizes, so I don't want a left small and a right large.  

I tried to set an automatic discount for this using Shopify's native discount functionality and applied Buy X get Y + percentage to the first combination (small + left and right).  When trying to save the next combination for the same product (standard + left and right) Shopify blocks this saying that only one automatic discount at any one time (error message:  "Only one automatic discount can be active at a time. Select another date.")

Has anyone found a solution for this?  

If the only solution for this is another app, then do you know one that costs USD5 / month or less?  Most apps seem to cost more than my Shopify subscription ... 

Many thanks.