Well that happened to me to. Here was my chat with them concerning it.
Chat started on 29 May 2017, 03:32 AM (GMT+0)
(03:32:34) *** VGK joined the chat ***
(03:32:34) VGK: signed up for prime on 27/5 before 6pm but no voucher
(03:32:36) ezbuy : Hi! FREE Agent fees 28-31 May 2017 2300 *T&C apply. Welcome, we are here to assist you.
(03:32:36) ezbuy: ANNOUNCEMENT: Prime Annual membership FREE cash voucher. This offer is valid only for selected customers who have receive this email invitation and free Cash Voucher will be deposited in accounts on 25th May. This cash voucher will be withdrawn before 28th May if customers are not eligible for the offer.
(03:34:06) Customer Service: Due to high volume of request in queue, we apologize for keeping you waiting. Our operators are busy at the moment, please leave us a message, and we'll get back to you shortly.
(03:35:18) *** Julian joined the chat ***
(03:35:22) Julian: Hi
(03:35:23) Julian: Dear customer, kindly hold on for a second, as our system seems to be having some connection issue. Kindly provide me a little more time to check. You may look for us later to check for you, as the IT department is currently resolving the connection issue to our server. Sorry for the inconvenience caused
(03:35:51) VGK: oh is this normal?
(03:36:01) VGK: lots of people not getting their voucher?
(03:37:44) Julian: I can provide you an answer if I am able to check it for you. But we are having some connection issue from the main server, I'm afraid I can't check it for you now.

Whereas we are indeed having some problem with customer not receving their voucher, but generally it would be due to these customer are not the ones randomly selected by our system.
(03:37:46) Julian: receiving*
(03:38:24) VGK: If thats the case then why did i recieve the email stating I was invited
(03:39:08) VGK: I really do not wish to sign for prime if there wasnt the voucher
(03:40:12) Julian: Unfortunately, due to certain issue, customer that were not selected by the system had also received the email. Sorry about that.
(03:41:03) VGK: if that is the case you will need to refund those who didnt receive the voucher
(03:41:21) VGK: i dont wish for prime anymore
(03:42:03) Julian: For selected customer, they will receive the voucher before the subscription, and they may use it after subscribing. For your account, i'm afraid it isn't selected for this promotion. I'm afraid that you won't receive the voucher as it is issued immediately for selected customer, sorry.
For this issue, we may only offer a 1 time special cancellation and refund your additional subscription if you would like to as it seems that you had received the email by certain error. For selected member, they will receive the voucher immediately. I'm afraid for your case, your account is not selected. Sorry
(03:42:20) Julian: Unfortunately, I am unable to make the amends for you now, as we are having some connection issue to the main server.
(03:42:32) Julian: Kindly hold on, and I will make the amends for you, once the server resumes in connection.
(04:03:56) Julian: Hi, sorry to keep you waiting. The server has resumed connection
(04:03:57) Julian: Can I have your user ID so that I can check for you?
(04:04:08) VGK: vgk
(04:04:27) Julian: Kindly hold on for a second
(04:31:57) Julian: Sorry to keep you waiting. Your subscription has been cancelled The 99sgd for subscription fee has been refunded to your account. We apologize for any inconvenience caused brought by this issue
(04:32:03) Julian: Is there anything else that I can assist you with?
(04:32:27) VGK: yes could u confirm the status of my withdrawing now
(04:32:35) VGK: its currently pending
(04:34:50) Julian: Unfortunately, that is with the finance department. I'm afraid to assist you with this, sorry
(04:34:59) Julian: Our finance department will follow up on the withdrawal
(04:42:45) Julian: This chat will be closed as there is no active conversation . Do feel free to contact us again if you need any further assistance. *Please do rate our service and we promise that we will improve to serve you better!
(04:46:19) *** Julian left the chat ***
(04:46:49) *** VGK left the chat ***
Their fault becomes like my fault for believing their email sent to my account