Some more details, that explain why it’s not so simple…
One of the principals that we hold dear from a design/UX point of view, is that what you see in your feed should add up to your current balance.
We could have simply made our feed generator not create a feed item for Coin Jar transactions, but this would break that principal.
Good is the Enemy of Great
Instead what we want to do is have feed items represent transactions, and transactions representing one or potentially multiple movements of money. So one transaction could represent both the card payment, and the movement of money into your coin-jar.
Once we do this you’ll see that your purchases really are “rounded up”… So spending £3.50 at Pret, will appear as £4- (Pret), with a subscript (50p also saved to your coin jar).
This piece of work is something we call Asynchronous Transactions and is a big ticket item on our road map. It enables many many more cool things 
We’ll make the Coin Jar of your dreams once this piece of work is done.