Only Flexible packages can be refilled. Both Fixed and Flexible packages can be cancelled.
Flexible package: Yes! You can refill it at any time while it is running. The speed stays the same and the package runs longer. The same setup fees apply to every refill, so a 100 USDT refill adds 90.21 USDT to your balance.
Fixed package: No. Once purchased, a Fixed package cannot be changed.
Yes, both Fixed and Flexible packages can be cancelled at any time: in My Packages, open the Action menu (•••) next to the package and choose Cancel order. Mining stops and you get back the unspent balance B, together with the 3% buy fee charged on it:
R = B / (1 − 0.03)
For B = 50.00 USDT: R = 50.00 / 0.97 = 51.55 USDT
The infrastructure fee and the fixed fee are not refunded. Earnings not yet paid out are paid to your wallet right away.