Translation:The sausage costs nine kronor and fifty öre.
To add some clarity 50 öre or more is rounded up and <50 öre is rounded down, and it is the final sum that is rounded. If you pay with credit card you will pay the exact amount. Lets say you buy two chocolate bars at 9,95: if you pay with cash it'll cost you 20 crowns and if you pay by card it'll be 19,90. Note the decimal comma usually used in Sweden.
To be a little more specific about the punctuation: we use the decimal comma in maths, but prices are usually written with a colon*. So 9/2 = 4,5 when writing in Swedish (not 4.5 as in English) but something would cost 4:50. Prices/money sums in whole numbers are written like 4:-
In time expressions, we use a period: 4.50 means 'ten to five'.
* It's ok to write prices with a comma, but using a colon is more common. For decimal numbers in general though, using a period is wrong in Swedish and can lead to misunderstandings.
Regarding the use of comma or colon in prices: just about every receipt I could find around my desk (which is quite a few) used a comma, so at least when it comes to receipts it's a common practice. The only exception I could find actually used the decimal point, although that is most certainly not kosher.
By rounding up or down when paying cash and paying the exact amount by card, is this a way of encouraging people to use cards and get rid of cash altogether? Is there a charge for paying for small amounts by credit cards - here in England, in some shops there is a charge added for paying by credit card for anything less that £5 for example. Also some purchases charge extra when bought on line.
It's probably just a byproduct of having had 50-öre coins up until only very recently. More commonly, prices are put as e.g. 9.99 instead of 10.00 as is common virtually everywhere.
Many smaller shops will indeed enforce a low minimum charge amount, or add some minor sum (maybe 5-10 kr) if you don't shop for more. It's technically against their terms with the bank and/or credit card company, but everybody looks the other way.
I might add, by the way, that Sweden may be the single most card-friendly country in the world. Card acceptance is everywhere, and a lot of people - especially in younger generations - virtually never use cash.
You can try asking in the general Swedish forums, but I don't think Duolingo is a good platform for learning slang. Any guide to it we write would be half obsolete in a few years anyway. Personally, I think the only way of really learning slang is immersion - spending time with natives and picking it up through trial and error.