Hi Jack, Some Flat Fee providers could have a log-in system where you can make changes. nevertheless they need to input these changes into the MLS, which only the Broker has access to ... so the answer is yes, always the Flat Fee Listing Broker will need to upload your changes you the MLS system regardless of how you get these changes to them.
Most (if not all) MLS systems require that you have a valid executed listing agreement with the Broker. That is OK and better for you because this will list all the fees you must pay and the services that will be provided. make sure that there are no additional fees other than your Flat Fee Listing fee. It will also explicitly show the offer of compensation for the Buyer's agent. This is a document to protect you from any future surprises. Make sure that there is no cancellation fees, that changes are free and that you can post open houses as well (this is our business model, but we are not in Illinois yet).
Be sure that the broker is properly licensed in your state and that he/she shows you samples of the listings they have; as questions: how will potential buyers contact me? will you change a fee to the buyer or buyer agent? - (this is a red flag if the answer is yes). How long will it take for changes to be reflected in the MLS? How many pics can I post? Are there any other fees other than the BUyer's agent commission and the Flat Fee listing?
You will be fine! Just choose the company that give you the best gut feeling, the one that has the best (and truthful) reviews and NEVER deal with a referral service - deal directly with the Broker and ask to talk to the broker directly. goo rresponsible brokers take calls from prospect customers.