Instead of creating a Charge Item as you might typically do for extra fees, for a Transportation fee with limited capacity, you would need to create a separate Program and attach a Class with a capacity limit. Here is how:

Create a Charge Item on the Items page by clicking on the blue plus button. Add a " Transportation " charge or whatever you want to call it. Make sure it is "required" and "excluded from Payment Plans" and any other options you require.

Create a Program Option/Program Name called "Transportation" or something under "Program Options."

Add a Program and fill in the Program Options you just created:

Under the Enrollment Model, choose Simple Pricing and the days the transportation runs:

If transportation is free, leave the pricing on "Free." If there is a cost, choose "Fixed Cost" under Program Cost and enter the amount of the fee:

Decide if you want specific instructions/descriptions on the Enroll Form at enrollment. This can be created under the section "Enroll Form." This message will override any Program instructions on the My Company page.

This is how the overriding message looks on the Enroll Form when someone enrolls in the Transportation Program:

Head to the Classes page and click "Add a Class" to create a Class within the newly created Transportation Program (it can be the same or a different name). You can add an Instructor (or not).

Be sure to add your capacity. This is how many Enrollees can be transported for the year. You can optionally add a start/stop time and or dates.

That's it! Now, Enrollees can enroll in your Transportation Program!