"I can't help but buy = 我非买不可"
I thought this phrase is self-explanatory. Perhaps not and seems not that commonly used though it is a legitimate expression.
I even asked my daughter who had 12 years of full time formal Chinese language studies in a Chinese speaking environment whether she has heard it before, and replied "no, but I know what it means"
In any case, the questioner asked for a Chinese expression that has the same impression as "can't help but..." So I suggested the above phrase which I don't consider "advanced" or difficult, maybe a bit classical sounding, but that's about all, and therefore let it stood on its own in answering the question.
So, what is the phrase all about?
Just as in English, often times a double negative phrase has greater communicative impact than a positive assertion, like "I can't get no satisfaction", (from the song "Satisfaction"), "I hardly have no money", and the all time favorite, "You ain't seen nothing yet"
我非买不可 is a Chinese equivalent of a double negative.
我 = I,
非 = not,
买 = buy,
不 = not,
可 = can.
So literally, "I not buy not can"
Or, sensibly, "I cannot not buy"
Hope that helps.
BTW, you can use this handy phrase in all sorts of situations, even in everyday social conversations to sound "elegantly refine"
Just substitute the 3rd verb with whatever activity you have in mind, like, 我非吃不可，etc.