That's if you're applying for an extension, but you're saying you already have. You can't apply for another extension whilst you're still under consideration for one which is pending
It's not a multi-entry add on, it's called a re-entry permit. You can get it at the airport. Cost is 1000 baht for single, 3800 for multi plus they charge 200 baht for form/photo. EDIT. I just saw Tod's comment. Appears you don't need form and photo now.
It is time consuming but you have to remember the covid extension is a concession, it's not a standard process, so more complicated procedures are often instigated. Some people are asked to show flight tickets leaving the country. When you apply for a covid extension you basically have to adhere to the conditions.