Self-driving trucks have been discreetly delivering cargos between Arizona and Texas under an unpublicized pilot project launched by UPS in collaboration with TuSimple, the delivery giant revealed on Thursday, as it announced making a minority investment in the autonomous trucking startup.

UPS said it had acquired minor stakes in TuSimple as it bids to cut costs and save time to improve the efficiency of its ground deliveries further. TuSimple claims it can reduce delivery cost by 30%. No financial details of the transaction were disclosed, and neither company commented on the size of the investment.

The more substantial chunk of the announcement tackled the pilot program, which it said took off in May around the same time the United States Postal Service and TuSimple launched a two-week pilot program of the same nature under which TuSimple’s autonomous trucks delivered mail for the postal service between Phoenix and Dallas.

During the first phase of the pilot program, the TuSimple trucks were guided by an engineer and a safety driver to ensure its efficacy and safety. The program is expected to continue until UPS reaches Level 4 autonomous driving where it will be allowed to leave the wheel alone during deliveries.