Mailgun offers a Flex plan where you have the first 1000 emails for free. You can find more details here https://help.mailgun.com/hc/en-us/articles/360048661093-How-does-PAYG-billing-work-
To get the Flex plan, you must upgrade to Foundation Trial (30 days trial). Within this period, you’ll be able to downgrade to the Flex plan. Mind you, you won’t see that you are on the Flex plan until your 30-day trial expires. Five days prior to downgrade you’ll see this notice in Plans & Billing section
While in the trial period, you’ll be able to set up your domain (or subdomain) and set SPF & DKIM records. That will prevent your emails from ending up in the spam folder.
Hope this helps.