When building a bundle with a fixed price is it possible to add on optional items that increase the value of the box before check out?

Yes, you can achieve this if you use the "Start Price" feature in the app. So for example your fixed price would be the start price of $100 and all the items with prices on the next steps would be added on top of the starting price.
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The "Start Price" is applied automatically.
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Prices of the items chosen are added on top of the start price
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