non-numeric value warning

I have a website shop (www.heidiline.com) using WordPress eCommerce, and whenever anyone attempts to purchase something and goes to the shopping cart/checkout, there are at least 8 of these messages all over the page: ‘Warning: a non-numeric value encountered in /home2/heidilin/public_html/wp-content/plugins/wp-e-commerce/wpsc-includes/cart.class.php on line 987.’ Needless to say, folks don’t want to purchase anything when they see this. I would appreciate any help on this. Thank you!

By the way, to temporarily resolve this, I dialed back the PHP to version 7.0 and that seems to work. This problem only occurs with versions after 7.0

The page I need help with: [log in to see the link]