Disembarking ship in Rouen and seeking best way to transfer 6 adults and 6 pcs luggage to Paris apartment in 4th agrand. Van fine. Prices seem to vary hugely.
Best. Based on what criteria? Your options include the frequent direct 2 1/2 hr Flixbus to the Bercy Seine station for under €15/pp; the hourly 1 1/2 hr trains to Gare St Lazare for around €25-30/pp; a hired van and driver from a company like Daytrip for over €300, or renting your own van which would need to be a large capacity to accommodate all of the luggage, and if automatic transmission, could cost over €400 for the day. If opting for bus or train, you would probably want to get 2 taxis to get to your accommodations from the station.
FYI, I believe you mean your accommodations are in the 4th arrondissement. It's abbreviated arr.
apartment in 4th agrand.
4th arrondissement perhaps? You could all take a train to Paris Gare St Lazare and then by taxi . I have not priced it out, but start there and compare.
In my opinion the easiest would be to schedule a door to door transfer with Daytrip. It will definitely cost considerably more than public transportation, so it will not be "best" if price is a consideration. It all depends on whether or not spending the extra money is worth the convenience to your group.