FedEx Corp. has told rival UPS Inc.'s customers that any business from UPS that FedEx onboards by March 31 will receive priority treatment from FedEx regardless of whether the Teamsters union strikes UPS or not.

In a letter to various UPS shippers, FedEx (NYSE: FDX) said parcels tendered by the end of the month can be assured of receiving high-priority service levels. FedEx urged UPS (NYSE: UPS) customers to take advantage of the near 30-day window to ensure they have committed capacity in the event of a labor disruption.

"Don't wait until it's too late," the letter said.

The message from the email is that FedEx cannot guarantee it can prioritize volumes for UPS shippers unless the business is received by March 31. In a presentation accompanying the email, FedEx said shipments from UPS customers received after March 31 will be handled on a first-come, first-served basis.

This is not FedEx's typical engagement style. Generally, the company treats issues like this in a low-key manner. However, according to Nate Skiver, head of consultancy LPF Spend Management LLC, FedEx's pitch isn't shocking. 

FedEx needs volume and revenue, Skiver said in a LinkedIn post Friday. In addition, it's a rare chance for the company to publicly exploit a UPS weakness, namely concerns about 350,000 UPS Teamsters members walking off their jobs by Aug. 1, the day after the current five-year contract expires, according to Skiver.