Inventory Source recommends the use of the UPS shipping module.  It is simple and easy to use.  It is weight based which you have for your dropship products.

However, if you still need USPS, then this guide will help you to apply your settings for your osCommerce USPS Shipping Module.
You will need to have registered an account with USPS.  You can find the webtools for USPS at:
www.usps.com/webtools/
In the left column you will see "Access USPS Web Tools".  This is where you sign up for your account.  Most guides tell you that you must first complete a test of the connection for your account using the test server option on the USPS shipping module in your store Admin.  Some also recommend changing the sort order for the USPS from "0" to "1".

While there is still the server setting for a test in the module options, the test no longer appears needed for the USPS module.  You just have to sign up at www.uspswebtools.com. You will receive your webtool username/password via email. Then call the following number and mention you're using osCommerce:
The ICCC is staffed from 7:00AM to 11:00PM Eastern Time.
Telephone: 1-800-344-7779
E-mail: icustomercare@usps.com

They will activate your account over the phone. Remember to make sure your details are added to the admin of your store, and change the setting to the production server.

Some possible errors that you might have from the USPS Shipping Module are:
Error 1:
"United States Postal Service
An error occurred with the USPS shipping calculations.
If you prefer to use USPS as your shipping method, please contact the store owner. "
This error often occurs if you have not contacted the ICCC to activate your osCommerce CRE Loaded USPS shipping module.  (See above for the contact details.)
 
Error 2:
"United States Postal Service
This Information has not been included in this Test Server."
IThis error means that the User ID that USPS has for you is not matching what you are sending them. It could be a type in your Admin or on your account with USPS.  You can continue to check your store settings and code, if you are unsure of where or how your osCommerce site was installed.  You might find a quicker resolution by contacting USPS directly and verify the details in your USPS account and your store Admin settings. (See above for the contact details.)

If you still need a merchant account service, please see our most recomended provider E-Commerce Exchange.  They offer excellent support when setting up your site.

Please see below to learn how to apply the Authorize.Net payment module to your ZenCart site.

1- Log into the ZenCart administrator's panel.
2- Click the Modules menu. You will see a page with all the default payment modules. 3- Select Authorize.net and click the Install button on the right.
4- On right side of the next page you will see a small form for this payment module.  Here are the settings to apply to use Authorize.Net:

Enable Authorize.net Module: answer - True
Login Username: Type the login username used for the Authorize.net service
Transaction Key: This is the transaction Key used for encrypting TP data
Transaction Mode: Unless you want to make some tests first, you may set it to production mode
Transaction Method: set to credit cards
Customer Notifications: If you wish AN to send notifications to your customers, set this to true
Payment Zone: We recommend you to leave that to none to accept payments from all zones.
Set Order Status: Set this to default if this is the primary method of payment.
Sort order of display: Lowest is displayed first - you may leave that 0

5- Please remember to select the Update button at the bottom when you are done to apply your changes.  Also, it is always good to run a test order on your site to see how it works from the customers' perspective.  Make sure you have set payment and shipping options on your ZenCart site for your dropship wholesale store.

Some dropship resellers on Zen Cart want to offer free shipping for orders above a certain amount.   If your dropship products have MAP or are in a very competitive product segment, then free shipping can help you to make a sale or encourage bigger orders when selling dropship wholesale products.  Who doesn't love something for free?  Inventory Source can help you manage even larger volumes of wholesale products on your Zen Cart site which will allow you to better leverage your Zen Cart traffic and really drive sales.

1-      From your ZenCart admin menu, mouse over the Modules menu, and select the Order total option with a click.

2-      From the list which appears on the next screen, click the Shipping module and then the Edit button on the right menu.

3-      Set the minimum amount for free shipping in field (examples: Enter "100" for $100, enter "75" for $75, etc.)

4-     Click Update button at the bottom of the box your changes to be applied to your ZenCart site.