I travel a lot and can not rely on USPS mail. How could I substitute an e-mail address for a USPS physical address, with respect to a legal matter in Florida small claims court?
You don't need to. E-mail is the default method of service in Florida, so you'd actually need to notify the court if you wanted to use anything else.
The starting place is Florida Small Claims Rule 7.080(b):
How Made. When a party is represented by an attorney, service of papers other than the statement of claim and notice to appear shall be made on the attorney unless the court orders service to be made on the party. Service on ... a party not represented by an attorney must be made in compliance with Florida Rule of Judicial Administration 2.516.
So if you have a lawyer (you should) all service would be made on your lawyer, who would forward them on to you automatically or at least upon request.
If you were not represented by counsel, you'd follow the reference to Florida Rule of Judicial Administration 2.516. Then jump to subsection (b)(1):
Service by Electronic Mail (“e-mail”). All documents required or permitted to be served on another party must be served by e-mail, unless the parties otherwise stipulate or this rule otherwise provides.
And then to (b)(1)(C):
Service on and by Parties Not Represented by an Attorney. Any party not represented by an attorney may serve a designation of a primary e-mail address and also may designate no more than two secondary e-mail addresses to which service must be directed in that proceeding by the means provided in subdivision (b)(1) of this rule. If a party not represented by an attorney does not designate an e-mail address for service in a proceeding, service on and by that party must be by the means provided in subdivision (b)(2).
So it looks like you would probably want to see if you can sign up for the court's e-filing system, which would automatically result in service to your e-mail address. If you can't use that system, you would just file a designation of e-mail address. Leon County has a template available online.
There may be some variation among different courts, though, so I'd recommend verifying all of this with an attorney.