The current setup has APT, Unit, or Suite numbers on a 3rd line, however, sometimes customer don't put apt or anything and just a number. This will show on the 3rd line. Sometimes this can be an issue with USPS because it reads from bottom of the address and up. So it will think it's a street number with no street and may cause delays or issues with delivery. I've gotten returns from address like these before.

name
Street address
APT #
City, State Zip

Here is the USPS address format info page:
http://pe.usps.com/businessmail101/addressing/deliveryAddress.htm

This particular line is important to note as USPS recommends NOT to have it below the street address:
"If you can’t fit the suite or apartment number on the same line as the delivery address, put it on the line ABOVE the delivery address, NOT on the line below."

I've been having to move it manually after the street on the same line or move or move it to address line 1 and swap it with line 2.

If you have used Endicia's standalone app, it will actually AUTO CORRECT and move it after the street on the same line or above the street on the 2nd line, but it will NEVER keep it below the street line. Shipping Easy has stated "The label format for USPS labels comes directly from our Labelserver, Endicia." However, again if Endicia app can auto correct, shippingeasy should be able to take the info and program it so that this can be self corrected as well.

It's not to say that it won't be delivered, but It's just less chances for delays or undelivered mail and I'm sure there's a reason why Endicia themselves does it for their app as well.