Would you interpret this as an offer or an inivtation to treat? Provided Platinum members did fill in the attached form and return it in time?

I know an offer is bout substance not form, so saying "offer" doesnt necessarily mean much, but i feel as though due to it being directly sent to certain members it is more an offer? Any help appreciated :)

"On 10 February 2020, Rockport sent a letter by registered post to all of its 22 platinum members. The letter stated:

Dear valued member,

As a platinum member, we make an exclusive offer to you. You can get a return business class flight from Perth to Hawaii via Sydney during June - December 2020 for only 500 dollars. All you have to do to accept this exclusive offer, is fill in the attached form and return it to our Perth CBD office by 5pm WST 9 May 2020."