Compare the phrases "я заказал вам номер" and "я заказывал вам номер". The first one is about the very fact of booking, and the speaker is informing someone that this job is done. When someoone is using the form "заказывал," the emphasis is not on the booking itself - it's just a phrase that's used as a link to what follows next, as in:
Я заказывал вам номер, но вы по прилёте лучше позвоните и уточните.
So imagine that your sentence has a second part, such as:
- Я заказывал у вас номер [и вот теперь я хочу заселиться].
- Мы заказывали у вас столик [и вот мы пришли].
There's nothing stopping you from using the perfective form in such cases, but the de-facto imperfective is also used.
А заказывал я примерно месяц назад. - Точную дату не помните?
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– CocoPop Mar 04 '20 at 17:02