We just recently (twice) rented from both Avis & Budget at their location's on
99 Southeast 2nd Street, SE 1st Ave at, Miami, FL 33131...
They share the same building and are very close to the port and offer a comp shuttle to and from POM. No need for the hassle at MIA.
If you're a Costco member check their website...this is where we get the best rates. Even though at times MIA is a few $'s cheaper than the offsite offices it's not worth the hassle and then paying for transportation to get there.