The English is 'normally received WITHIN 10 days' - this seems to be a perfect situation for use of 'd'ici', but this was not indicated as correct.
This sentence in English means something along the lines of 'you may receive the article any time, but you can expect it by 10 days from now'. In other words, don't start ringing us to ask about it until after at least 10 days.
I am not convinced that either 'en' or 'dans' as described in the lessons are better fits. It is not a statement that delivery/receipt 'takes 10 days' to happen, or that delivery/receipt will occur on the 10th day.
