I agree. I was going to say that the English simple past in "you bought it" doesn't call for the perfective aspect of Daje'pu', but I agree the perfective is a better translation of the meaning in this case than a literal translation would be. But would it be even better to put the protasis in the perfective too? Daghorpu'chugh Daje'pu
Alternatively, departing further from the literal translation but a more literal meaning, which seems more suitable in Klingon: Daghorchugh DaDIlnIs (If you break it, you must pay for it... if my grammar is right)
Klingon perfective does not equal English perfect, and English simple past does not exclude Klingon perfective. You bought it describes a done deal, something that has already happened and is done, so -pu' is entirely appropriate here. (This Duolingo course mangles the differences between Klingon aspects and English tenses rather badly.)
I considered putting -pu' on the Daghorchugh, but decided against it on the grounds that in the very moment that you can say Daghor you break it, you are in the middle of the act of breaking it, and it's at that moment that you have already bought it. So, while it's true that Daghorpu'chugh Daje'pu' (if you have already broken it you have already bought it), the sentence is meant to convey Daghorchugh Daje'pu' (if you break it right now you have already bought it).
Your grammar is correct in Daghorchugh DaDIlnIS. I don't see how this has a more literal meaning or seems more suitable in Klingon. If you're thinking of "You'll pay for this!" as a sort of threat, I suggest not using caricature as a guide to what Klingons "would" say. The saying You break it, you bought it isn't a threat; it's a statement of policy.
I wasn't intending in any way this to be a comment on supposed Klingon aggression - more the directness of their language. The intention of the policy is that the person breaking the object must pay for the damage. Whether or not they take possession of the broken object or not is of little consequence. I assume "You break it, you buy/bought it" is formulated as such because of the alliteration and rhythm, rather than the policymaker being committed to the transfer of ownership of the broken object. That's why I considered DIl a more appropriate translation of the intent of the policy than je'.
Most sentences in this course are given in both directions, eventually. The English to Klingon option on this exercise has been disabled for the very reason you mention. We want the "you can also use" to show that the most literal translation is not always the best translation. Later in the course we will actually make you translate such "mismatched" sentences in both directions so that you can get used to thinking about the meaning of the sentence rather than the word for word translation.