The BuyMerchantItem() blizzard function does not seem to support buying more than one stack at a time of items.

When I have < 1 full stack, the addon will purchase up to 1 stack fine, but not 2 or 3 stacks.
When I have >= 1 full stack < 2 full stacks, it will buy up to 2 stacks, but not 3.
When I have >=2 < 3 full stacks, it will successfully buy 3