There is no variable in v8 and above for the Special Delivery Instructions but the following workaround should allow you to use the 'User Definable' field on the invoice page for this option as this is automatically included in the customer email.
* go to the 'Design' menu and select 'Text'
* click on the 'Go To' button in the bottom left corner of the window
* set 'Phase' to 0 and 'ID' to 14 and click 'OK'
* this will take you to the 'Invoice User Definable Field'
* tick the box under the 'Show' column for this field
* enter the text for the prompt into the 'Current Value' field (ie, Enter any special delivery instructions)
* click 'OK'
* go to the 'Design' menu and select 'Library'
* on the 'Layouts' tab, expand the 'Checkout Area' group (click on the '+')
* double-click on the 'Invoice Address Fields' layout
* locate the 'Invoice User Defined' variable and 'cut' it from this layout
* click 'OK' to save and close the layout
* still in the 'Library | Layouts | Checkout Area group', double-click on the 'Shipping Phase' layout
* locate the 'Shipping User Definable Prompt' variable and comment it out by putting '<!--' at the beginning and '-->' at the end of it and paste the 'Invoice User Defined' variable that you cut from the other layout, so it looks something like:
<!--Shipping User Definable Prompt-->
Invoice User Defined
* click 'OK' to save and close the layout
* update your site.
If you want to move the position of this instruction on the online receipt then look for the 'InvoiceUserDefined' variable in the 'Receipt Invoice Address Details' layout and in the customer email you should look for the 'ShipUserDefinedOnlineXML' variable.
* go to the 'Design' menu and select 'Text'
* click on the 'Go To' button in the bottom left corner of the window
* set 'Phase' to 0 and 'ID' to 14 and click 'OK'
* this will take you to the 'Invoice User Definable Field'
* tick the box under the 'Show' column for this field
* enter the text for the prompt into the 'Current Value' field (ie, Enter any special delivery instructions)
* click 'OK'
* go to the 'Design' menu and select 'Library'
* on the 'Layouts' tab, expand the 'Checkout Area' group (click on the '+')
* double-click on the 'Invoice Address Fields' layout
* locate the 'Invoice User Defined' variable and 'cut' it from this layout
* click 'OK' to save and close the layout
* still in the 'Library | Layouts | Checkout Area group', double-click on the 'Shipping Phase' layout
* locate the 'Shipping User Definable Prompt' variable and comment it out by putting '<!--' at the beginning and '-->' at the end of it and paste the 'Invoice User Defined' variable that you cut from the other layout, so it looks something like:
<!--Shipping User Definable Prompt-->
Invoice User Defined
* click 'OK' to save and close the layout
* update your site.
If you want to move the position of this instruction on the online receipt then look for the 'InvoiceUserDefined' variable in the 'Receipt Invoice Address Details' layout and in the customer email you should look for the 'ShipUserDefinedOnlineXML' variable.