While the WPS-Preparation automatically reloads the configuration notecards when it detects an inventory change, the WPS-MainScript does not. If you need to reload the configuration (for example because you changed something in the .menus notecard, you need to manually restart it. A convenient way to do so is to drop the script WPS-ResetMain in your object's inventory. This particular (full perm) very simple script will send a Reset signal to the WPS-MainScript and then remove itself from the object's inventory.
Do not remove the ".positions" notecard or WPS-ConfigServer script when you sell your products because the .positions notecard will be read again when the owner changes. Removing the ".menus" notecard should be fine, but unnecessary.
When using a WPS-powered attachment, the final customer is able to adjust the position and orientation of the attachment, based on what is defined in the ".positions" notecard. This positioning can be done by the usual edition tools in the viewer, and the script will automatically retain this setting for each pose.
This feature does not replace the ".positions" notecard.
In recent versions of the main viewers (Firestorm et al.), a new menu allows an avatar to adjust their vertical position with respect to the ground or the prim they are standing on. This may (will) affect the attachment too.
The menu will normally stop working after 30 seconds. This is a well-behavior measure that aims at keeping the lag low. This timeout value can be modified as of version 3.0 by the TIMEOUT directive.
If a particular menu has more than 9 buttons, "previous" and "next" buttons will appear as required to allow navigation between pages of 9 buttons.
You must pay attention to the attachment point of course, although the WearPose System will technically work from any attachment point (including the HUD). The best choice for a seat or couch is probably the "avatar center" (sometimes called "root") because this point is unaffected by playing animations of the avatar.
Do you need more features ? Feel free to contact me.
Many thanks to Carisma Alex for her unconditional support.