Since it's a commission build, your best route is to gauge your customer's interest in add-ons relative to the cost.
PE? Barrel? I highly doubt your buyer would want the added expense of these add-ons (plus your additional labor). I wouldn't even bring in aftrmkt tracks whatsoever.
If I were you, I'd avoid the discussion about add-ons completely. It's a business deal. You don't want this:
"the basic tank is $100 but it'll be $200 if I want PE and a metal barrel? What are those? $300$ if I want 'froo-ill' tracks? Are you crazy?"
All the while he fears you're ripping him off. Don't do it. Nor should you undersell the value of the service you provide. Commission build buyers want a nice desk/bookcase ornament, that's all.
Since you have little knowledge of PE/Barrels/tracks for this model, I suspect you're not a long-time model builder. Even all the more reason to deliver a simple product, at a fair price, in order to assure a happy customer.
To be frank, if someone were to inquire of me to provide a top level, well detailed and painted Tiger 1 commission build, I'd charge $200-300 and provide him with a DML Tiger 1 kit. Well detailed and not needing more add-ons.