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.
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.