Jeff,
The printing service should only need to know the size at which you want it printed. At that point they can adjust the PPI to a number that will shrink the print size, but not destroy any pixels.
You can adjust the DPI as you Export if you wish, but that should not be necessary. To change the DPI, choose JPEG Original, say, and click on Edit and you can change the DPI to a higher number, and export.
If you export any photo, open it in Preview, click on Tools and choose Adjust size then you can see the impact of changing the Resolution on inches/or metric measure, which is what the printer can do. In Adjust Size be sure to Deselect the option to Resample -- the latter would otherwise remove pixels and result in a lower resolution.
Ernie