Hi Rafal,
Would you consider licensing an existing solution on a royalty fee basis, at least until you build volume required, rather than purchasing\licensing the software from one of the software companies.
We operate an online webshop that sells personalised printed products (calendars is one of our many items) and we are looking at ways we can increase our revenue streams by possibly working on a royalty fee basis (for use of our front end & back end software + server) for companies in non competing markets.
If this is something of interest to you send me a private message and we can take this up further.
Kind regards
Theo