If you want extra goodies like the web interface, all you need is an upgrade key.
BUY NOW WITH PAYPAL OR CREDIT CARD · 9,99€
BUY NOW WITH BITCOIN · 9,99€
If the automated options don't suit you, maybe DM a reseller on Discord:
For Cashapp, Etherum, Litecoin, and other payment methods, contact 333#1337 on Discord or visit their shop.
If you're planning to buy in bulk via Bitcoin, contact Sainan#2020 on Discord.
Once you've got your upgrade key via Sellix (who'll send you an email) or Discord, you just need to copy it and click Paste Upgrade Key in-game.