Invoicing-how much?

Hello forum members,

I need your advice on invoicing a client. I have attached a file, if it helps in determining the scale of the Artwork. this client also happens to be a friend who owns a publishing company.

I have to invoice for three bookmark designs (pharmaceutical company). two designs being straight forward to put together, as the look/colours were kinda already specified, though the files did go back and forth three times before final print as the client wanted small changes. The 3rd design, had a little more to it. I created a design proposal, which was approved and then prior to printing got binned. they asked for a redesign to include a new image (the train), which meant a new design. Anyways, I can't charge them an hourly rate as it took me much longer than an experienced designer to complete the job (to point out, I have not done Graphics design work for 7 years now) my computer skills are a little rusty. not quite up-to speed with the shortcut keys- meaning I worked many loooooong hours. If I charge them per item/ per design...what can u suggest is a reasonable amount to invoice in euros....ANYONE ON THIS FORUM FROM EUROPE?????

I don't like to throw them off completely as they are offering more work which may change if they receive an invoice that they are not happy with.. I roughly calculated 50€ per bookmark at 15€/hr...Is it too much? ( I based on the rate I was at, 7 years ago) YOUR 2 Cents, so appreciated.