Typing into billing name mirrors the value into shipping name.
Copy targets may be choice controls: a radio checks its matching value in the target set, a checkbox mirrors its own value's checked state. Click a checked source size again, or press Space on it, to deselect it, and its mirror clears too.
Donation and fee sum into the total as you type. Display and transport are different jobs: the <output> shows the total — a total in an editable-looking box invites editing — and a hidden input posts it. Both carry data-fs-amount-total, and the engine reconciles both at load, not just after the first edit.
<output>
data-fs-amount-total
Card year offers the next 5 years; class year reaches 15 years back and 10 forward — offsets run in either direction; card month offers the next 12 calendar months.
data-fs-amount with a value separates the price from the answer: a unit price multiplies a numeric answer, a choice or a file charges flat when made, and a negative amount is a discount. Bare data-fs-amount still reads the control's own value as the amount.
data-fs-amount
A term counts toward the total only while it is actually in play: an unchecked choice contributes nothing, and a term drops out the moment its field becomes irrelevant.