Seems it's totally free if you cancel your trial after March 3. Enjoy :)
The offer may be redeemed from (12:00 AM) on 3 March 2019 to (11:59 PM) on 10 March 2019. To be eligible for the $20 coupon offer, you must sign up for a free 30-day trial of eBay Plus during the Offer Period. If you cancel your free 30-day trial of eBay Plus before 3 March 2019, you will no longer be eligible for the offer. Customers who have previously signed up for eBay Plus and cancelled their membership prior to the Offer Period must sign up for a paid membership of eBay Plus during the Offer Period to be eligible for the offer. Current eBay Plus members are not eligible for this offer.
The $20 coupon will be sent to you via email and/or eBay Message Centre on 3 March 2019.
The offer entitles you to $20 off the purchase price (excluding postage costs) on all items listed on www.ebay.com.au except the Excluded Items when you spend $100 or more in one transaction during the offer period. A maximum of 1 transaction applies.
Multiple items may be purchased in one transaction (up to a maximum of 10 items per transaction). All monetary amounts specified in these terms and conditions are in Australian dollars (AUD).
Excluded Items means all items listed in the following categories: Cars, Bikes, Boats (9800), Car & Truck Parts (6030), Coins (11116), Gift Cards (184609), Other Lots More Items (88433), Real Estate (10542), Services (316), Tickets, Travel (11730); and all items sold by the following Sellers: Officeworks, Fantastic Furniture, Target.