Danielle from the Shopify Guru Team here.
I would recommend either of these two places:
Let me know if you have any other questions!
Free trials are only available during the original 14 day trial period. However, if you add on the retail package and do not like it, you can remove it at any time. Shopify also prorates our billing, so if you remove the retail package halfway through a billing period, that will be credited to your account.