It’s not much different from setting the starting bid for a normal item.
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Once you have added the Instant Item to your auction, you can customize the starting bid. You will notice you cannot set the starting bid lower than the reserve price plus the buffer (the buffer is intended to cover the Stripe transaction fee plus some additional revenue for your organization to ensure you make a profit). You can set the starting bid to be higher than the reserve price to ensure your organization makes the desired minimum amount of profit off of this item.