The kinds of comments that are useful are less about what the code already tells you is happening, and more things like:

/* we don't use the actual price but the discounted price, as per email from Manager Bob on 2022-09-16 */

subtotal += price * customer_discount_factor;

or

/* note there's a 2ms delay while relays settle; this is subtracted from sample time, so timeout is not what you might expect */

select(0,&readfd,NULL,NULL,&timeout);