web-services/bandcamp: Add snippet for total purchase value

docs/web-services.md

@@ -27,6 +27,14 @@
     :::js
     [...document.querySelectorAll('#track_table .track_row_view')].map(el => `${el.querySelector('.track_number').innerText} ${el.querySelector('.track-title').innerText} (${el.querySelector('.time').innerText})`).join("\n")
 
+### Show total purchase value
+
+Works on https://bandcamp.com/USERNAME/purchases
+The result needs to be feeded into an intelligent calculator like qalc.
+
+    :::js
+    Array.from(document.querySelectorAll(".col.purchases-item-total div")).map(el => {const raw = el.children[1].innerText.replace("\n", " ").replace(",", ""); return raw.substr(1, raw.len)}).join(" + ")
+
 ## Ototoy
 
 ### Get title listing (for MusicBrainz)