All trains go through Caen, many requiring a change there. If you can get a PREM'S fare for Paris-Bayeux, leave France as the default country, do NOT choose cancellation insurance, and print the ticket yourself. (Choosing cancellation insurance requires giving an address, and SNCF doesn't recognize addresses in North America.)
While you can get a PREM'S fare for times that require changing in Caen, Paris-Caen is the PREM'S leg and Caen-Bayeux is a regional train leg. Therefore you CANNOT print the ticket but must have it mailed to an address in France. (Obviously, this won't work in your case.)
There are two ways around this. (1) Book only the Paris-Caen leg online and print the ticket. Buy a Caen-Bayeux ticket when you get to Gare St-Lazare, or (2) Pick a direct train to Bayeux. Print the PREM'S ticket. The lowest PREM'S fare (15€) is for direct trains.)