Try to assign configured store when making new purchase batch

so the UI logic doesn't need to do anything special for that

rattail/batch/purchase.py

@@ -63,6 +63,22 @@ class PurchaseBatchHandler(BatchHandler):
         return self.config.getbool('rattail.batch', 'purchase.allow_expired_credits',
                                    default=True)
 
+    def make_basic_batch(self, session, **kwargs):
+        """
+        Create a "bare minimum" batch object.
+
+        Here we'll set the default store, if none is provided.
+        """
+        # try to assign store for new batch
+        if 'store_uuid' not in kwargs and 'store' not in kwargs:
+            store = self.config.get_store(session)
+            if store:
+                kwargs['store'] = store
+
+        # continue with usual logic
+        batch = super(PurchaseBatchHandler, self).make_basic_batch(session, **kwargs)
+        return batch
+
     def should_populate(self, batch):
         # TODO: this probably should change soon, for now this works..
         return batch.purchase and batch.mode in (self.enum.PURCHASE_BATCH_MODE_RECEIVING,