Travel from Seville to Lagos, Portugal

What about renting a car in Seville and driving to Lagos? Is crossing the border with a rental car a problem??
No. The problem arises with the supplement that the car rental companies charge for one-way rentals where the car has to be 'repatriated' to the country of origin. A car with Spanish plates has to be returned to Spain if you drop it off in Portugal and that costs money(unless it's at a border point.)