Unlike some carriers, United does not offer discounts for pre-paying baggage fees. For a US domestic ticket, the price is $30 for the first bag and $40 for the second.
https://www.united.com/web/en-US/content/travel/baggage/changed-bag-rules-optional-services.aspx
You will be asked to pay for your bags when you check in. So, if you check in online, that's when you pay. If you wait and check in at the airport, that's where you will pay.
As you plan to use the Resort Airline Check-in Service (RAC) on your return, you'll need to follow their policies. As noted, you'll receive instructions. But, you probably will have already checked in (via the website or app) and paid for your luggage by the time you get to the RAC location.