Can I route customer to a specific page after checkout?

Is there a way to route customers to a specific page after PayPal checkout? I need to link to a custom uploader after they checkout through PayPal (my only payment module). Is this possible? And if so, how? I have to use the custom uploader because my client wants customers to upload large files and they were not happy with Zencart's uploader not showing the upload progress.

Sure it can be done. But I guess its better and maybe easier to replace the zencart uploader with your custom uploader.

Ok, and how would I do that?

Have a loook into /includes/modules/attributes.php

around lines 456 - 468 you will find this code, that draws the input fields for the file upload,

// iii 030813 added: support for file fields
                  if ($products_options_names->fields['products_options_type'] == PRODUCTS_OPTIONS_TYPE_FILE) {
                    $number_of_uploads++;
                    if (zen_run_normal() == true and zen_check_show_prices() == true) {
                      // $cart->contents[$_GET['products_id']]['attributes_values'][$products_options_name['products_options_id']]
                      $tmp_html = '<input type="file" name="id[' . TEXT_PREFIX . $products_options_names->fields['products_options_id'] . ']"  id="' . 'attrib-' . $products_options_names->fields['products_options_id'] . '-' . $products_options_value_id . '" /><br />' . $_SESSION['cart']->contents[$prod_id]['attributes_values'][$products_options_names->fields['products_options_id']] . "\n" .
                      zen_draw_hidden_field(UPLOAD_PREFIX . $number_of_uploads, $products_options_names->fields['products_options_id']) . "\n" .
                      zen_draw_hidden_field(TEXT_PREFIX . UPLOAD_PREFIX . $number_of_uploads, $_SESSION['cart']->contents[$prod_id]['attributes_values'][$products_options_names->fields['products_options_id']]);
                    } else {
                      $tmp_html = '';
                    }
                    $tmp_html .= $products_options_details;
                  }

Depending on the implementation of you custom code, you replace parts of this code with your custom code. Loos at other places, that are involved in the file upload.