Note to self: this is a simple, tested way of storing a document in the db:

declare default element namespace "http://www.tei-c.org/ns/1.0";
declare namespace  xdb="http://exist-db.org/xquery/xmldb";
declare namespace util="http://exist-db.org/xquery/util";

let $doc := <doc><test>My test doc</test></doc>,
$coll := collection('/db/coldesp/oai/records/')
return xdb:store($coll, 'test.xml', $doc)

This will delete a document:

return xdb:remove('/db/coldesp/oai/records/', 'test.xml')

This snippet will delete a document if it exists, then replace it:

let $remove := 
	if (fn:doc('/db/coldesp/oai/records/test.xml')) then
           xdb:remove('/db/coldesp/oai/records/', 'test.xml')
	else (),
$doc := <doc><test>My test doc 2</test></doc>,
$coll := collection('/db/coldesp/oai/records/')
return
xdb:store($coll, 'test.xml', $doc)