The answer you cited discusses the fees that can be charged on fiat currencies by gateways, when exchanging currencies, and when using other users to swap IOUs. The XRP fees are imposed by the network itself.
The transaction fee is the only fee that is destroyed. Currently, it is 10 drops per transaction under normal load conditions. (A drop is a millionth of a Ripple.)
The fees you mentioned are actually reserves. Reserves are amounts of XRP you must hold to maintain an entry in the ledger. The reserve is returned when the entry is removed, however, there is no way to remove an account. The reserve is not destroyed, it stays in the account. You can use your reserve to pay transaction fees. You just can't create a new entry if you don't have the reserve to cover it.
Currently, the only entries that require a 50 XRP reserve are credit lines and offers. It is expected that future entries, such as contracts, will also require a reserve. A credit line only counts towards your reserve under some circumstances. Other people can't extend credit to you and increase your reserve.