diff --git a/README.md b/README.md
index b719a7b..02aad4b 100644
--- a/README.md
+++ b/README.md
@@ -55,8 +55,8 @@ The buyer can have the following keys
 
 ### Items
 
-The list of items should contain maps with two required keys (:vat and :title), and a key
-providing the cost of the item. The price can be provided in one of three ways:
+The list of items should contain maps with a required :title, an optional :vat (if not provided it is assumed that
+item is VAT free), and a key providing the cost of the item. The price can be provided in one of the following ways:
 
  * :netto            - is a set price and will be displayed as provided
  * :hourly           - is an hourly price - JIRA will be queried in order to work out how many hours should be billed
@@ -69,6 +69,10 @@ providing the cost of the item. The price can be provided in one of three ways:
                        be :base. Otherwise the final price will be scaled accordingly. This is pretty much equivalent
                        to working out what the hourly rate should be in a given month and multiplying it by the number
                        of hours worked in that month
+ * :function         - an S-expression describing how to calculate the net value. Only numbers, basic mathematical
+                       operations (+, -, /, *) and timesheet specific variables are supported (:worked, :required,
+                       :to, :from).
+
 
 Examples:
 
@@ -81,6 +85,9 @@ Examples:
     ; 23% VAT, working part time with a base salary of 5000
     {:vat 23 :base 5000 :per-day 4 :title "Part time job at 5000"}]
 
+    ; 23% VAT, with a custom function
+    {:vat 23 :function (* :worked (/ 10000 :required)) :title "Custom function"}]
+
 
 ### Credentials
 
diff --git a/resources/config.edn b/resources/config.edn
index 4933f07..6cd1748 100644
--- a/resources/config.edn
+++ b/resources/config.edn
@@ -9,6 +9,8 @@
           :nip 9875645342}
   :items [{:vat 8 :netto 123.21 :title "Buty kowbojskie"}
           {:vat 21 :hourly 43.12 :title "Usługa szewska"}
+          {:netto 321.45 :title "Usługa szewska bez VAT"}
+          {:vat 23 :function (* :worked (+ 1 2 3 (- 23 13))) :title "Pucowania obuwia"}
           {:vat 23 :base 4300.00 :per-day 4 :title "Praca za ladą"}]
   :font-path "/usr/share/fonts/truetype/freefont/FreeSans.ttf"
   :credentials {:tempo-token "5zq7zF9LADefEGAs12eDDas3FDttiM"
diff --git a/src/invoices/core.clj b/src/invoices/core.clj
index d54c06f..ad95d61 100644
--- a/src/invoices/core.clj
+++ b/src/invoices/core.clj
@@ -9,25 +9,36 @@
 (defn invoice-number [when number]
   (->> [(or number 1) (-> when .getMonthValue) (-> when .getYear)] (map str) (str/join "/")))
 
-(defn calc-part-time [when creds {base :base per-day :per-day}]
-  (let [{worked :worked total :required} (prev-timesheet when creds)
-        hourly (/ (* base 8) (* total per-day))]
+(defn parse-custom [work-log func]
+  (cond
+    (and (list? func) (some #{(first func)} '(+ - * /))) (apply (resolve (first func))
+                                                                (map (partial parse-custom work-log) (rest func)))
+    (list? func) (throw (IllegalArgumentException. (str "Invalid functor provided: " (first func))))
+    (some #{func} '(:worked :required :to :from)) (func work-log)
+    :else func))
+
+(defn calc-part-time [{worked :worked total :required} {base :base per-day :per-day}]
+  (let [hourly (/ (* base 8) (* total per-day))]
     (float (* hourly worked))))
 
-(defn calc-hourly [when creds {hourly :hourly}]
-    (-> (prev-timesheet when creds) :worked (* hourly)))
+(defn calc-hourly [{worked :worked} {hourly :hourly}]
+    (* worked hourly))
+
+(defn calc-custom [worked {function :function}]
+  (parse-custom worked function))
 
 
-(defn set-price [when creds item]
+(defn set-price [worked item]
   (cond
-    (contains? item :hourly) (assoc item :netto (calc-hourly when creds item))
-    (contains? item :base) (assoc item :netto (calc-part-time when creds item))
+    (contains? item :function) (assoc item :netto (calc-custom worked item))
+    (contains? item :hourly) (assoc item :netto (calc-hourly worked item))
+    (contains? item :base) (assoc item :netto (calc-part-time worked item))
     (not (contains? item :netto)) (assoc item :netto 0)
     :else item))
 
 (defn for-month [when {seller :seller buyer :buyer items :items creds :credentials font-path :font-path} & [number]]
   (pdf/render seller buyer
-              (map (partial set-price when creds) items)
+              (map (partial set-price (prev-timesheet when creds)) items)
               (pdf/last-working-day when)
               (invoice-number when number)
               font-path))
diff --git a/src/invoices/pdf.clj b/src/invoices/pdf.clj
index 429bdd6..6758c86 100644
--- a/src/invoices/pdf.clj
+++ b/src/invoices/pdf.clj
@@ -8,7 +8,7 @@
   (float (/ (Math/round (* val 100.0)) 100)))
 
 (defn vat [{netto :netto vat-level :vat}]
-  (round (* netto (/ vat-level 100))))
+  (if-not vat-level 0 (* netto (/ vat-level 100))))
 
 (defn brutto [{netto :netto :as item}] (round (+ netto (vat item))))
 
@@ -22,7 +22,11 @@
 (defn format-product [{netto :netto vat-level :vat title :title :as item}]
   (concat
    [[:cell {:colspan 4} title]]
-   (map str [1 (-> netto round str) (str vat-level "%") (vat item) (brutto item)])))
+   (map str [1
+             (-> netto round str)
+             (if-not vat-level "zw." (str vat-level "%"))
+             (-> item vat round str)
+             (brutto item)])))
 
 
 (defn get-title [team who which]