First Check Firmware Package Price with Link , Then Choose the Package , if Package price is 17 USD then Buy Place Order for 17 Credits.
( Same Like That if You want a package which price is 20 usd then buy 20 Qty to Active Package. ) if Package Price is 35 USD Then Buy 35 Qty.
Firmware Support Site Introduce New Packages time to time , So whatever package price , Buy same Amount of Credits Qty.