Helene, edit /includes/modules/shipping/usps.php to locate its install function. You'll see a section of code like the following (which is K11i):
$db->Execute(
"INSERT INTO " . TABLE_CONFIGURATION . "
(configuration_title, configuration_key, configuration_value, configuration_description, configuration_group_id, sort_order, set_function, date_added)
VALUES
('Shipping Methods (Domestic and International)', 'MODULE_SHIPPING_USPS_TYPES', '0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, 70, 0.00, 0, .21875, 0.00, 0, 4, 0.00, 0, 4, 0.00, 0, 66, 0.00, 0, 4, 0.00, 0, 4, 0.00, 0, 20, 0.00, 0, 20, 0.00, 0, 66, 0.00, 0, 4, 0.00, 0, 70, 0.00, 0, 70, 0.00', '<b><u>Checkbox:</u></b> Select the services to be offered<br><b><u>Minimum Weight (lbs)</u></b>first input field<br><b><u>Maximum Weight (lbs):</u></b>second input field<br><br>USPS returns methods based on cart weights. These settings will allow further control (particularly helpful for flat rate methods) but will not override USPS limits', 6, 0, 'zen_cfg_usps_services([[B]\'Media Mail Parcel\', \'USPS Ground AdvantageTM\', \'Priority MailRM\', \'Priority MailRM Flat Rate Envelope\', \'Priority MailRM Legal Flat Rate Envelope\', \'Priority MailRM Padded Flat Rate Envelope\', \'Priority MailRM Small Flat Rate Box\', \'Priority MailRM Medium Flat Rate Box\', \'Priority MailRM Large Flat Rate Box\', \'Priority Mail ExpressRM\', \'Priority Mail ExpressRM Flat Rate Envelope\', \'Priority Mail ExpressRM Legal Flat Rate Envelope\', \'First-Class MailRM International Letter\', \'First-Class MailRM International Large Envelope\', \'First-Class Package International ServiceTM\', \'Priority Mail InternationalRM\', \'Priority Mail InternationalRM Flat Rate Envelope\', \'Priority Mail InternationalRM Small Flat Rate Box\', \'Priority Mail InternationalRM Medium Flat Rate Box\', \'Priority Mail InternationalRM Large Flat Rate Box\', \'Priority Mail Express InternationalRM\', \'Priority Mail Express InternationalRM Flat Rate Envelope\', \'USPS GXGTM Envelopes\', \'Global Express GuaranteedRM (GXG)\'[/B]], ', now())"
);
Don't pay attention to the various ' elements (they're needed for the database insert), but those strings identify the various USPS shipping methods supported by the shipping module.