Actually, I see one definite error as well as a runon sentence. The error is the inclusion of the word "that" near the end of the sentence. It should be removed ("....difficulties in these areas can affect......."). By including "that" and ending the sentence as shown, there is an incomplete thought and "that" is never fully explained. The sentence is too long. There either needs to be a period after "...positive impression" or, less favorably, a semi-colon. Also, and this is personal preference, I would replace "how" with "that". "Remember how" implies the retailer has had experience with difficulties in these areas, whereas "remember that" implies the retailer is aware that difficulties in these areas can lead to problems.

you don´t need the "that" in this sentence, the sentence should be "On the other hand, a lack of queues, favourable credit facilities and efficient staff all lead to a positive impression, so retailers should remember how difficulties in these areas could affect a sale or even lose a customer permanently." you can´t say "retailers should remember how difficulties in these areas that can affect a sale" because you have only one subordinate clause and "how" is already the relative pronoun, you can´t use "that" together with "how".

Other than being a long sentence, the grammar is excellent. I would break it into two sentences where the word "so" is. "...positive impression. Retailers should..."

The word that should be excluded. It leaves the second part of the sentence without a predicate.