### Summary
- Downgraded the project from PHP 8.1+ to be compatible with PHP 7.4.
- Replaced modern PHP features with backwards-compatible equivalents.

### Changes
- Updated `composer.json` to target PHP 7.4 and adjusted dev-dependencies (`phpunit/phpunit` ^9.5, `laravel/pint` ^1.13).
- Added `marc-mabe/php-enum` to `require` and converted all PHP 8.1 `enum` classes to `MabeEnum\Enum` classes.
- Removed typed properties from all classes (e.g., `protected USPS $usps` to `protected $usps`).
- Removed union types (e.g., `object|array`) and replaced them with PHPDoc `@return` or `@param` annotations.
- Replaced PHP 8.0+ functions like `str_starts_with()` with PHP 7.4 compatible `strpos() === 0`.
- Updated test suite to remove PHP 8 return types and adjusted assertions to work with broader PHPUnit 9.x versions.

### Verification
- Ran `php -l` on all source and test files to ensure syntax compatibility with the current environment (which is backwards compatible for PHP 7.4 syntax).
- Verified that all modern PHP features (Enums, Union Types, Typed Properties, modern string functions) were removed or replaced.