How to add to a user_meta field (append)

by

Use get_user_meta():

$original = get_user_meta( 
    $user->ID, 
    'purchase_history', 
    TRUE 
);

echo $original . ' Pancake Breakfast 05/15/2005';

To update the value use update_user_meta():

update_user_meta( 
    $user->ID,  
    'purchase_history',   
    $original . 'Pancake Breakfast 05/15/2005' 
);