In my customized "confirmOrder.asp code, I display the options selected by the user from an ASP.net page.

I set the option values by hardcoding the ID of the optionId and it's value which places in the cartoptions nicely.

Example:

<form name="CustomQuote" action="cart.asp" method="post">
<input type="hidden" value="add" name="mode" />
<input type="hidden" value="customquotes.aspx" name="frompage" />
<input type=hidden value="custom" name="id" />

<input type="hidden" value="264" name="optn0" />
<input type=hidden value="<%=strFontColorName%>" name=voptn0 />

<input type="hidden" value="40" name="optn1" />
<input type=hidden value="<%=strFontName%>" name=voptn1>

<input type="hidden" value="18209" name="optn2" />
<input type=hidden value="<%=strFontSize%>" name=voptn2 />
<input type="hidden" value="<%= strCustomPrice %>" name="" /> <!-- I want to pass the Option Price Diff -->

<input type="hidden" value="18008" name="optn3">
<input type=hidden value="<%=strChars%>" name=voptn3 />

<input type="hidden" value="19050" name="optn4">
<input type=hidden value="<%= strCustomXXX&".pdf" %>" name=voptn4 />

<p align="center"><input type="submit" value=" Submit Order " name="CustomQuote"></p>
</form>

In the above example I hope I can pass the price difference to the cart.asp as an 'option' value set in the coPriceDiff field in cartoptions.
if there is not a price diff option name then who can I best set the price difference value?

Vince and all:
I figured out a way to change the price of the product in the cart.
I pass the price as a session value from my asp confirm page to the cart.
In inccart.asp sub addproduct(theid), I look for theid = my product name and...

' CustomQuote Price 31 July, 2019
if (theid = "custom") then
addedprods(3,numaddedprods) = session("customPrice")
end if

Voila.

Just an FYI - I offer a rush service at checkout and it's actually a product that's hidden.
When customers choose this, it's added to the customers cart with javascript and ajax, so no edits to the core files are needed, and no need to leave the shipping options page.
The same could be done with pricing.

I mention this because it sounds like maybe you are editing the inccart.asp script.