A family of Microsoft relational database management systems designed for ease of use.
One option I don't see mentioned is that you don't necessarily have to replace Access at all.
I've helped a lot of businesses migrate the data to SQL Server while keeping the existing Access front end for in-house staff. If you have 20 years of forms, reports, queries, and VBA that already work well, there's no reason to throw all of that away.
Once the data is in SQL Server (whether it's hosted on Azure, Winhost, or another provider), you can build a browser-based application for your sales agents while your office staff continues using Access. Both applications share the same data. It's often the most cost-effective approach because you preserve your existing investment while adding modern web access where it actually provides value.