What grammar makes 同 品 項 第 2 仵 6折 mean "Buy one, get the second at 60% of base price"? So in the convenience store, there were bottles of iced coffee with the slogan:

同  品  項 第 2 仵  6折  

I was told that this meant: "Buy the first item, get the second item at 60% of base price."

I was able to find the individual characters in various dictionaries:

同  tong2 be the same

品 pin3 merchandise

項 xiang4 item? expenditure?

第 [dì] number

2

件 [Thanks to wilson and user238264 for the correction]

6

折 [zhé] discount

The hard parts are the last two Mandarin characters.  I couldn't find a translation for 件  and while "lose money" roughly fits "discount" for 折 [zhé], I couldn't see any grammar.

Possibly there isn't any grammar to be found.  Possibly the convenience store is wildly abbreviating its Mandarin because most customers can infer huge amounts of meaning from context.

Any grammar and usage insights are welcome. Thanks.
I am pretty sure that you have the second to last character wrong. It makes much more sense as 件 (item).  Also, 折 should be read as zhé in this context.  The character has a large number of meanings, but in this context it means discount (and the single digit numbers preceding it count by 10%; this is a common idiomatic construction for expressing discounts).  Finally, 第 signifies that the following number is to be read as an ordinal (1st, 2nd, 3rd, ...) 

So, a very literal translation of 同品项第２件６折 would be:
same product, second item, discounted to 60%