diff --git a/15.md b/15.md
index f9e4b790..5e3164e7 100644
--- a/15.md
+++ b/15.md
@@ -8,7 +8,7 @@ Nostr Marketplace (for resilient marketplaces)
 
 > Based on https://github.com/lnbits/Diagon-Alley
 
-> Implemented here https://github.com/lnbits/nostrmarket
+> Implemented in [NostrMarket](https://github.com/lnbits/nostrmarket) and [Plebeian Market](https://github.com/PlebeianTech/plebeian-market) 
 
 ## Terms
 
@@ -54,7 +54,7 @@ A merchant can publish these events:
         {
             "id": <String, UUID of the shipping zone, generated by the merchant>,
             "name": <String (optional), zone name>,
-            "cost": <float, cost for shipping. The currency is defined at the stall level>,
+            "cost": <float, base cost for shipping. The currency is defined at the stall level>,
             "countries": [<String, countries included in this zone>],
         }
     ]
@@ -63,9 +63,12 @@ A merchant can publish these events:
 
 Fields that are not self-explanatory:
  - `shipping`:
-   - an array with possible shipping zones for this stall. The customer MUST choose exactly one shipping zone.
+   - an array with possible shipping zones for this stall.
+   - the customer MUST choose exactly one of those shipping zones.
    - shipping to different zones can have different costs. For some goods (digital for example) the cost can be zero.
    - the `id` is an internal value used by the merchant. This value must be sent back as the customer selection.
+   - each shipping zone contains the base cost for orders made to that shipping zone, but a specific shipping cost per
+   product can also be specified if the shipping cost for that product is higher than what's specified by the base cost.
 
 **Event Tags**:
 ```json
@@ -87,17 +90,30 @@ Fields that are not self-explanatory:
     "price": <float, cost of product>,
     "quantity": <int, available items>,
     "specs": [
-      [ <String, spec key>, <String, spec value>]
-     ]
+        [<String, spec key>, <String, spec value>]
+    ],
+    "shipping": [
+        {
+            "id": <String, UUID of the shipping zone. Must match one of the zones defined for the stall>,
+            "cost": <float, extra cost for shipping. The currency is defined at the stall level>,
+        }
+    ]
 }
 ```
 
 Fields that are not self-explanatory:
  - `specs`:
-   - an array of key pair values. It allows for the Customer UI to present present product specifications in a structure mode. It also allows comparison between products
+   - an optional array of key pair values. It allows for the Customer UI to present product specifications in a structure mode. It also allows comparison between products
    - eg: `[["operating_system", "Android 12.0"], ["screen_size", "6.4 inches"], ["connector_type", "USB Type C"]]`
 
-_Open_: better to move `spec` in the `tags` section of the event?
+    _Open_: better to move `spec` in the `tags` section of the event?
+
+- `shipping`:
+   - an _optional_ array of extra costs to be used per shipping zone, only for products that require special shipping costs to be added to the base shipping cost defined in the stall
+   - the `id` should match the id of the shipping zone, as defined in the `shipping` field of the stall
+   - to calculate the total cost of shipping for an order, the user will choose a shipping option during checkout, and then the client must consider this costs:
+     - the `base cost from the stall` for the chosen shipping option
+     - the result of multiplying the product units by the `shipping costs specified in the product`, if any. 
 
 **Event Tags**:
 ```json
@@ -211,4 +227,4 @@ Customer support is handled over whatever communication method was specified. If
 
 ## Additional
 
-Standard data models can be found here <a href="https://raw.githubusercontent.com/lnbits/nostrmarket/main/models.py">here</a>
+Standard data models can be found <a href="https://raw.githubusercontent.com/lnbits/nostrmarket/main/models.py">here</a>