Game Time and Subscriptions are completely separate things, people.
Subscriptions are reoccurring and require a bill, so you cannot use Battle.net money to purchase them. Game Time, however, is something completely different. This is what many people use Battle.net money cards to purchase.
Open "Shop" via your launcher or from the Battle.net website.
You will see a list of the available games with icons on the left side, below that you will see a text list. You want to select "Game Services".
On the next page select "Game Time".
That will bring you to this page
All of the options on this page are purchasable with Battle.net funds.
Purchasing a 90 day Game Time card will leave you with $3.03 in your bnet balance. (Requires 3 Tokens)
Purchasing a 180 day Game Time card will leave you with $12.06 in your bnet balance. (Requires 6 Tokens)
If you want to sub for an entire year, purchasing two 180 day Game Time with battle.net cash will save you $24.12. (Requires 11 Tokens, so $15 of the $24 can stay in gold value)
There are ways you can do this to go for the lowest balance remaining, but I chose the method to give me the most money left over for pets/mounts/card packs/loot crates/future expansions.