A retainer is not typically open. You would normally provide a quote based on a minimum amount of hours and either A)Bill that amount if they’ve used it up or not. B)Have some sort of agreement to roll over hours to be used in the future within a max amount of time.
Generally I bill retainers at the start of the month BEFORE the work is done, so there is no issue being paid and they have an incentive to use the hours they already paid for. You can also make an arrangement for any hours used over and on top of the bucket of hours to either carry forward for some time (perhaps you may have a surplus/deficit of +/-10%) to be credited and accounted for quarterly or end of a project/cycle.
The general point of a retainer is some sort of guaranteed amount of work from your end, while giving them a priority service and rate in exchange.
What’s the issue selling skills for a couple of hours? If your rates are in place, that shouldn’t be an issue.
R