Legal Question in Criminal Law in Florida

No. Paying for travel is perfectly acceptable.

Paying reasonable travel expenses is perfectly fine. Often there are special fees for expert witnesses, which is also OK to pay.

Of course, there is always an argument to be made that a "witness is being paid for his testimony" when an expert is involved, but aren't the lawyers being paid, and the judge, and the clerk and the bailiff. Just because someone is being paid, doesn't necessarily mean their testimony is less-believable.