I enter the number 18,879.86 the form transforms the number to 1,887,986. But when i round up to 19,000 it accepts the number with no problem, am i allowed to round up?

Sounds like you typed a comma instead of a decimal point.

As far as I can tell.you may round up.  Sometimes this is the only way to translate the true figure/amount into the return.

You can round up, but only to the next dollar. That number rounds up to 18,880 (not 19,000).