Calculate order end date based on date and number of weeks ACF fields

You could consider working with unix timestamps (a numerical representation of a date and time that is the seconds elapsed since 00:00:00 1st January 1970). This makes the math easier since we would be working with plain numbers:

// Convert pick up date to unix timestamp integer.
$start_timestamp = strtotime( get_field( 'pick_up_date' ) . ' 00:00:00' );

// Get the number of weeks as seconds:
// 60 - seconds in a minute, minutes in an hour.
// 24 - hours in a day.
//  7 - days in a week.
$weeks_in_seconds = get_field( 'number_of_weeks' ) * 7 * 24 * 60 * 60;

// Add the weeks period of time to our start timestamp.
$end_timestamp = $start_timestamp = $weeks_in_seconds;

// Convert our result timestamp back to a date string.
$end_date = gmdate( 'd/m/Y', $end_timestamp );