According to his service - That is, distribute them among the Levites as they may need them, giving most to those who have the heaviest burdens to bear.
Matthew Henry
Concise Bible Commentary
The offering of the princes to the service of the tabernacle was not made till it was fully set up. Necessary observances must always take place of free-will offerings. The more any are advanced, the greater opportunity they have of serving God and their generation. No sooner was the tabernacle set up, than provision is made for the removal of it. Even when but just settled in the world, we must be preparing for changes and removes, especially for the great change.
SDA Bible Commentary (limited)
Volume 1-7
5. Take it of them. The wagons and oxen were a voluntary offering (v.3) that must have been received gratefully by the Levites for the work of transportation. Moses apparently did not accept the offer until he was specifically authorized by the Lord to do so.