I like Suttons seeds, much prefer them to T&M ...
My local Wyevale has sold both (and Unwins) in the past. This year they seem to only have T&M and a few Fothergills and Sarah Raven.
Doesn't bode well for my annual seed-buy in their 50p-a-packet sale in September :(
If its cheapest free then you need to checkout multiple times 2-packets at a time, sorted into ascending price order ...
We sold our eCommerce software to companies doing "cheapest packet free" and grouped the items (by price) so that each cheapest one in a group was the freebie. None of the retailers though that was "fair" !!