Unfortunately you cannot trial a 365 license as Microsoft requires us as resellers to pay for licenses used up-front. As such our licensing with our clients is on a month to month, up-front basis. You can cancel your license at any time in order to not incur the charge for the following month.
Should you wish to trial an Exchange mailbox you we recommend selecting and Exchange Online 1 license as this is the most cost effective offer available allow for full Exchange functionality and a 50GB mailbox.