Here, I'll provide an example I generated with chatgpt with some prompting.

    type Server struct {
        Payment         services.PaymentService
        Storage         services.StorageService
        Quota           services.QuotaService
        Auth            services.AuthService
        Notification    services.NotificationService
    }

    // TransactionRequest is the request structure for a purchase
    type TransactionRequest struct {
        UserID    string  `json:"user_id"`
        AuthToken string  `json:"auth_token"`
        ProductID string  `json:"product_id"`
        Price     float64 `json:"price"`
        Currency  string  `json:"currency"`
        File      []byte  `json:"file"`
        Key       string  `json:"key"`
    }

    // TransactionResponse is the response structure after processing a transaction
    type TransactionResponse struct {
        Status  string `json:"status"`
        Message string `json:"message"`
    }

    // HandleTransaction handles the incoming transaction request
    func (s *Server) HandleTransaction(w http.ResponseWriter, r *http.Request) {
        var req TransactionRequest
        if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
            http.Error(w, "invalid request", http.StatusBadRequest)
            return
        }

        // Authenticate user
        userID, err := s.Auth.Authenticate(req.AuthToken)
        if err != nil {
            http.Error(w, "authentication failed", http.StatusUnauthorized)
            return
        }

        // Check and update quota
        allowed, err := s.Quota.CheckQuota(userID)
        if err != nil || !allowed {
            http.Error(w, "quota exceeded", http.StatusForbidden)
            return
        }
        s.Quota.UpdateQuota(userID, 1)  // Assume updating quota by 1 unit per transaction

        // Process payment
        transactionID, err := s.Payment.Charge(req.Price, req.Currency, "payment-token")
        if err != nil {
            http.Error(w, "payment failed", http.StatusInternalServerError)
            return
        }

        // Upload file to S3
        fileURL, err := s.Storage.Upload(req.File, userID + "-files", req.Key)
        if err != nil {
            http.Error(w, "file upload failed", http.StatusInternalServerError)
            return
        }

        // Send notification
        notificationMessage := "Your purchase has been processed successfully."
        s.Notification.SendNotification(userID, notificationMessage)

        response := TransactionResponse{
            Status:  "success",
            Message: "Transaction processed successfully: " + transactionID + " File uploaded to: " + fileURL,
        }
        w.Header().Set("Content-Type", "application/json")
        json.NewEncoder(w).Encode(response)
    }

It is still contrived obviously. But what I would like to show what happens if use stubs. You could pass anything into the dependency parameters and the test would pass. Mock allows you to "lock" in the implementation. Its tightly coupled yes but sometimes that is exactly what you want.

In this example that would be the userId, the bucket name, the key.

Mock allows you inject faults in the test setup. Lets say I want to test the error handling logic. I could do it with multiple types of stubs but if you squint its starting to look the same.

On the topic of Fake vs Stub vs Mock. Real or Fake is the ideal solution here but not always viable due to time/practical constraints. Stubs can be useful but don't give you fine grained control.