How to add additional costs to a product and add it as a sum charge in the cart

Hi Jason. Something must have gone astray in our thinking / operating here...

We added a new column called "AddGiftWrap" in the eCart Object columns section. We selected type "number" for this as nothing else seemed appropriate.

In the Add to Cart behaviour, in the Bindings section, our new column AddGiftWrap is there but the only "Updateable" options are "hidden field", "text field" and "select list". There is no option for checkbox. :o(

So...we proceeded with adding it as an updateable text field and then we manually changed it to a checkbox...
<input type="checkbox" name="BFcart_1_AddGiftWrap_Add" value="1" />

This, as you suggested, allows us to create the Charges rule but now, when we click the Add to Cart button we get this error, similar to an error we were previously getting:

Error Type:
Microsoft VBScript runtime (0x800A000D)
Type mismatch: 'cDbl'
/babyfish2013/WA_eCart/BFcart_VB.asp, line 34

Line 34 is:
retVal.AddGiftWrap = cDbl(itemAddGiftWrap)

We believe that when the checkbox isn't checked (when a customer doesn't require gift wrapping) then no value is being passed and, therefore, we are thrown the type mismatch error because it is looking for a numerical value.

Perhaps you can see a glaring error in all of this? Really appreciate your help. Thank you.
NJ