Well, expect printed.com I am not big fan of other 2. If you ask I will rank solopress.com on top when it comes to user experience and usability.
Honestly I don't consider myself competing these giants, so having something similar is not my requirement, mainly for the reason you have mentioned already "budget"
Completely get you - I'm not expecting you to be similar, but your system should be as easy to use. Print is very visual, if you're offering spot UV, foiling, letterpress etc - people want to see glossy images.
Printing.com/Exaprint - their backend systems are fantastic, I can't fault them - not talking from a print point of view though.
I recently purchased 1000x 400gsm cards, double sided, matt laminated
I paid £32.00 including delivery and VAT from one of the suppliers named above, we handled our own design.
The same spec is £78.00 on your site.
I don't expect you to compete - but I always speak to clients with a 'triangle' of Speed, Quality and Price - Anyone can only ever pick two of these.
In your case, you're more expensive, and take longer to deliver (5-7 days) - but I can't comment on the quality, so maybe yours is superior, but I can't fault my recent order from one of the above.
Can your customers instantly re-process an order through the back end of your site? eg, re-order another 1000 business cards, at the click of a button?
I feel that if you can make the entire system a joy to use - then you'll increase sales from pre-existing customers.
For new customers, and a conversion point of view - you need to be more visual, entice with offers on glossy looking products with high margin - embossing, spot UV, letterpress.