Buyee is a Japanese proxy shopping service, Mercari is a Japanese online marketplace somewhat like eBay. There are a lot of second-hand items. You can use google translate on chrome to go to the Mercari page and translate it so you can check the seller reputation and description.
5000 yen is about 53 AUD, with this you can get one smaller sized DHL shipment covered or a larger surface mail shipment. Surface mail is extremely slow with no tracking via a shipping container, it takes over 2 months to deliver. DHL takes less than one week with tracking but is very expensive particularly with volumetric weight.
Buyee is not the cheapest proxy service due to their fees and hidden currency conversion thats worse than Paypal. But with this coupon you can save.
To get this coupon you must have not made a Mercari order on Buyee before, and complete a Mercari order before April 30. Buyee takes 1-2 business days to actually submit your order during working hours. So don't leave it until the 30th.
Also the item you order can be easily purchased by someone else while you wait for Buyee to complete it.
The Coupon Usage Period is from the coupon distribution date - May 8, but this doesn't mean you have until then to make the order. You have to complete it before April 30.
Once your order arrives at the Buyee warehouse you can see the cost of each shipping method and apply your coupon.