One more thing:
I've assumed the portal field to be fairly stationary in space.
Any "portal station" will therefore either have to be a space ship if it wants to stay close to a portal field, or if it's a "regular" space station, it will orbit the star and therefore on average be its distance from the star away from the portal field at any given moment. Not that this matters massively, since you can still radio in your takeaway order ahead of time...
My main reason for complicating things this was is that many systems have more than one portal field anyway. Unless you envision each one having its own service station, it stands to reason one station could service them all - all portal fields in a multi-field system are (I assume) equidistant from each other and at the same distance from the star.
That's at least how it works in Kua system. Four portal fields close to the star, 3 AD* apart from each other (an useful snippet of info for freighters not stopping in the Kua that's just passing through, entering from portal and directly moving to exit from another).
*) Actually the portals (and hence, "my" portal station too) is at a listed distance from Kua star of 0,5 AD. Since a complete orbit of the star at that distance is 3,14 AD (the circumference of a circle is 2πr which in this case becomes just π = 3,14), one quarter of that is 0,8 AD. Since I don't want to increase the portals' distance away from the star, I suggest the "distance apart" figure is errataed, so that the sentence in question reads "In the Kua system, the portals are approximately 1 AD apart, which a bulk freighter travels in one day, but smaller ships much faster."