public class CheckbookAPI {
    
    // DigitalCheck__APIConfig__c is a custom setting object that stores the API Keys
    public static DigitalCheck__APIConfig__c Config = DigitalCheck__APIConfig__c.getOrgDefaults();

    public CheckbookAPI()
    {
    }

    public static String getChecks() {
        HttpRequest req = CheckbookAPI.getHttpRequest('GET', '/transactions');
        return CheckbookAPI.getHttpResponse(req);
    }    
    public static String createCheck(String email, Decimal amount, String firstName, String lastName, String semail, String description) {
        HttpRequest req = CheckbookAPI.getHttpRequest('POST', '/send_check');

        Map<String,Object> mapParams = new Map<String,Object>();
        mapParams.put('email', email);
        mapParams.put('amount', amount);
        mapParams.put('first_name', firstName);
        mapParams.put('last_name', lastName);
        mapParams.put('sender_email', semail);
        mapParams.put('description', description);
        req.setBody(JSON.serialize(mapParams));
        return CheckbookAPI.getHttpResponse(req);
    }
    

    private static String getHttpResponse(HttpRequest req) {
        Http http = new Http();
        HTTPResponse response = http.send(req);
        return response.getBody();
    }
    
    private static HttpRequest getHttpRequest(String Method, String Path) {
        // Initialize the request
        HttpRequest req = new HttpRequest();
        
        // Build the selected elements
        String SelectedElements = '';
        
        
        // Set the method
        req.setMethod(Method);
        SelectedElements += Method;
        
        // Set the Content-Type header
        if (Method == 'POST') {
            SelectedElements += 'application/json';
            req.setHeader('Content-Type', 'application/json');
        }
        
        // Set the endpoint
        String CompletePath = '/' + CheckbookAPI.Config.DigitalCheck__VersionAPI__c + Path;
        SelectedElements += CompletePath;
        req.setEndpoint(CheckbookAPI.Config.DigitalCheck__ServerURL__c + CompletePath);

        return req;
    }
    


}

global with sharing class DigitalChecksChargeController {

    public DigitalChecksChargeController(ApexPages.StandardController stdController)
    {

    }    
    
    @RemoteAction
    public static String processCharge() {
        return '200'; // Simulate the status code of the <charge> request
    }
    
    @RemoteAction
    public static String createCheck(String email, Decimal amount, String firstName, String lastName, String semail, String description) {    
        return CheckbookAPI.createCheck(email, amount, firstName, lastName, semail, description);
    }    
}

@isTest
public class CheckbookAPI_UsingStaticResources {
    public testmethod static void testWS() {
        String testBody = 'This is a test :-)';
 
        StaticResourceCalloutMock mock = new StaticResourceCalloutMock();
        mock.setStaticResource('Test_WS');
        mock.setStatusCode(200);
        Test.setMock(HttpCalloutMock.class, mock);
 
        CheckbookAPI  callout = new CheckbookAPI ();
        HttpResponse resp = callout.getChecks();  // <== GETTING ERROR : Static methods cannot be invoked through an object instance: getChecks()
        
        //System.assertEquals(200, resp.getStatusCode());        
        //System.assertEquals(resp.getBody(), testBody);
    }
}

Static methods cannot be invoked through an object instance: getChecks()