[inglor_cz]

True, but when moving stuff away from the Sun, perhaps you could use solar sails. Pressure of solar radiation is significantly higher at Mercury's orbit than in proximity of the Earth, so solar sails should be more efficient there.

Cargo, unlike humans, can take its time on the way. In case of minerals, it likely won't be damaged by vacuum or radiation exposure. The only thing that really interests you is that it arrives on schedule.