We offer a monthly seminar and I would like to accept fees for these on an á la carte basis. Can I do that?

Using bundles would be a great fit for this as it would allow customers on any membership level to purchase one or more seminars at any point in time. Using bundles will also allow you to send out targeted emails using the push notification system (Using Push Notifications) and also optionally add them to a specific mailing list with your email service provider so you can easily email those customers (Mapping Bundles to Mailing Lists).