Doing a dummy listing shows that Custom Postage is allowed, which means that your Large Letter at £1.55 is correct, so use it rather than the Simple being shown. That is post using a 2nd class large letter stamp, the only difference is you will not have any tracking.
I do this all the time, as I will never list anything that I cannot determine cost, and do not understand why buyer has been able to select a different method of postage.