Correct model naming.

pos_barcode_tare/models/pos_config.py

@@ -5,7 +5,7 @@ from openerp import models, fields
 
 class PosConfig(models.Model):
     _inherit = 'pos.config'
-    tare_label_button = fields.Boolean(
+    iface_tare_label = fields.Boolean(
         'Show tare label button',
         help="Print tare labels with this POS"
     )

pos_barcode_tare/static/src/xml/pos_barcode_tare.xml

@@ -36,7 +36,7 @@
     </t>
 
     <t t-name="TareScreenButton">
-        <t t-if="widget.pos.config.tare_label_button">
+        <t t-if="widget.pos.config.iface_tare_label">
             <span class="control-button">
             <i class="fa fa-print"></i>
             Print a tare label

pos_barcode_tare/views/pos_config_view.xml

@@ -8,7 +8,7 @@
             <xpath expr="/form/sheet/group[@name='features']" position="inside">
                 <separator string="Loose good options" colspan="4"/>
                 <group>
-                    <field name="tare_label_button" />
+                    <field name="iface_tare_label" />
                 </group>
             </xpath>
         </field>