This is a good deal for anyone who uses/needs Office software and the extra features in Microsoft Office 365 Business editions, but these costs a little more than the usual Office 365 Home Premium discounted cards in other posts which may suit most people's home needs and is great value with 5 PC's.
Microsoft 365 Business Standard is normally $20.60 per month, or $17.20 on annual commitment. This deal refunds the first three months after converting from trial so you may get >4-months free use. Minimum saving is $51.60 (3x$17.20)
Compare Microsoft 365 version features here: https://www.microsoft.com/en-au/microsoft-365/compare-all-mi...
I can't confirm if this offer/method works with other Microsoft 365 editions, but articles below indicate it might.
1a. (new users) Sign up to a trial of Microsoft 365. Log into an MS account, and sign up to the plan you want. You'll get a 1-month trial (at least - or get the offer immediately)
1b. (existing users) Sign into your Microsoft 365 Admin Portal at portal.microsoft.com
2. In the Admin Portal menu choose Billing > Your Products and see your subscriptions. Clicking "Cancel Subscription" link will trigger this pop-up offer https://imgur.com/a/HfcDw2t
3. Sign up to the 1-year plan, and it cancels the license you clicked on and switches your account across to a new billing license automatically. No risk of losing anything that way, and apparently they'll refund the first three months payments as they come out.
As the offer runs till 30-June-20 you could use a trial license first before signing up. I've used a trial for 7 weeks before as they keep the account active and nag for payment but don't stop things working (to avoid business disruption)
For users who don't want Windows/Mac versions of Office software, see this deal for 6-months free Microsoft 365 Business Basic https://www.ozbargain.com.au/node/532516
There are some other offers from Microsoft on other products I found while looking for details to link in the post.
Not a deal for everyone, but hope it's helpful for those who need Business features. Share any findings/feedback in comments and I'll update the post.