Actually ground freight---not to be confused with UPS Ground or something---should not be that high. It may take some time to get to you though. Perhaps it is already factored into the prices you are seeing? If not make sure you know how they will ship and what the cost will be upfront.
Do the online places you are considering have minimums? That could be a deal breaker.
Have you seen a sample and does the supplier promise to match it with similar marble or are they just going to ship you a box of some sort of marble? Make sure what you expect to get is well specified as far as thickness, surface, color and so forth.
If you were taking business from an independent flooring supplier in your area I would cringe a bit but most are gone and all left are national chains or box stores. They give nothing back to the communities in which they exist anyhow so...
If the numbers work, go ahead and order online.
You should, regardless of where you get the marble, order some extra tiles for repairs you may need down the road. You may not find the same color again.
Especially with marble, be sure and get floor tiles that are thick enough to hold up.