Since OPC 2.0.285, we added a special feature into OPC, which allows you to add a paid checkboxes into the checkout. You can use them for Charity, Product add-ons or any other case that you can imagine. These special checkbox products can be shown either within the totals or as a new products within the cart.
Configuration of the Checkbox products can be found at the bottom of the Shopper Fields config in OPC
You can use this section to add special products to the cart by clicking a checkbox. This can be used for product add-ons, charity donations or other purposes. Create a product with proper tax settings which is assigned to a hidden category or no category. Upon clicking the checkbox the product will be automatically added to the cart and will show either as a new product line (only on _german themes which are fully ajaxified) or as a new line within the dynamic totals (dynamic totals needs to be enabled at the tax tab). If you are not using an fully ajaxified theme or dynamic lines are not enabled, the new product will be only accounted in the subtotal and total value of the checkout's total. Within the order management or emails, the new product will be shown as a normal product. Please make sure you do not use any custom fields for this product. If you choose checkbox_products position, you can use echo $checkbox_products; within your onepage.unlogged.tpl.php or onepage.logged.tpl.php files. The checkbox is rendered with /components/com_onepage/themes/extra/default/checkbox_products.php which you can copy to your overrides directory of the OPC theme.