1) There is not built in multiple shipper option, but it can be accomplished with a little work.
the basic idea is to create a select list for the different shipping options, on the checkout page:
<select name="shippingOption">
<option value="FedEx">FedEx</option>
<option value="UPS">UPS</option>
</select>
Then on the confirm page, crate a hidden form element with the same name to capture the value that was posted:
<input type="hidden" name="shippingOption" id="shippingOption" value="<?php echo((isset($_POST["shippingOption"]))?$_POST["shippingOption"]:""); ?>" />
Using the eCart Set Session Value server behavior, capture the selected shipping option in a session variable.
Go to Window -> Server Behavior.
Click the plus button in the server behaviors window and select eCart -> General -> Set Session Value.
set the name to "shippingOption"
Set the trigger to any form submit.
for the value, click the lightning bolt and select the shippingOption form element.
Then apply the FedEx and UPS server behaviors to the confirm page.
the last step is to manually add an if statement around the code for each shipping type so it will only be used if it was selected:
<?php if((isset($_SESSION["shippingOption"]))?$_SESSION["shippingOption"]:"") && $_SESSION["shippingOption" == "FedEx"); { ?>
<fed ex code>
<?php } ?>
<?php if((isset($_SESSION["shippingOption"]))?$_SESSION["shippingOption"]:"") && $_SESSION["shippingOption" == "UPS"); { ?>
<ups code>
<?php } ?>
2) Yes, you can use your own design with eCart
3) The extensions needed to create the back end are DataAssist and Security Assist at a minimum. There is a tutorial for creating The admin back end on the Data Assist support page:
dataassist/
look in the how to section.