Creating a variable for the order subtotal (ie just the products and options) in the invoice:

1) Go to admin > store > pages.  

2) Click the edit for the invc page template.  

3) At the top of the page, click the link for Order Contents.  

4) Scroll down and locate the line
<mvt:foreach iterator="option" array="item:options">

5) Insert the two lines before and two lines after as you see below.

<mvt:item name="toolkit" param="math_multiply|lineprice|l.all_settings:item:price|l.all_settings:item:quantity" />
<mvt:item name="toolkit" param="math_add|basketsubtotal|basketsubtotal|lineprice" />

<mvt:foreach iterator="option" array="item:options">

<mvt:item name="toolkit" param="math_multiply|lineoprice|l.all_settings:option:price|l.all_settings:item:quantity" />
<mvt:item name="toolkit" param="math_add|basketsubtotal|basketsubtotal|lineoprice" />

6) The variable basketsubtotal will now exist and can be displayed or 
used in a link with &mvte:global:basketsubtotal;

7) If you need to add a negative coupon charge, for example, you can insert this into
the basket:charges foreach loop:

<mvt:if expr="'Coupon' CIN l.settings:charge:descrip">
<mvt:item name="toolkit" param="math_add|basketsubtotal|basketsubtotal|l.all_settings:charge:amount" />
</mvt:if>