Apply a custom price to an external product

I have an Invoice product I'm trying to import into the user's shopping cart - this product is dynamically created and priced, and it seems as if the Donation plugin would be perfect to get it in the system. However.. how do I use this plugin inside a custom component? What would I use to call it, and where would I put the product price?

Okay.. here's what I'm trying to do - I have an external component and it generates a price. I want to export that price into HikaShop onto the donation item and into their shopping cart. At the moment I have the code for the demo mostly working but it won't affect the product price.

Price: <input id="amount" type="text" value="<? print_r($orders[0][7]); ?>" readonly="readonly" onSubmit="hikashopToggleFields(this.value,'amount','invoice_order',0);" name="data[invoice_order][amount]"/>
Amount: <input id="hikashop_product_quantity_field_1" type="text" value="1" readonly="readonly" />
</td>
<td rowspan="2">
<input type="submit" class="button hikashop_cart_input_button" name="add" value="Add to cart" onclick="var field=document.getElementById('hikashop_product_quantity_field_1');if(hikashopCheckChangeForm('item','hikashop_product_form')){ return hikashopModifyQuantity('359',field,1,'hikashop_product_form','cart'); } else { return false; }">