I gather from your use of "we" that you have a team working on your startup. I think the answer to your question depends on whether the $15k is your only contribution to the startup or whether you've contributed $15k in addition to, e.g., intellectual property, sweat equity, etc.
If the $15k is your only contribution, while others on your team have contributed something of equivalent value to the startup, then I'm inclined to tell you to bear the costs as your fair share of getting this thing going.
If you've given $15k in addition to your IP and sweat equity, and your total contribution is therefore dispropotionately greater than that of your other founders, and it's not already reflected in your equity structure, then I don't think there's anything wrong with pointing out that discrepancy and asking how to address it. One way to address it might be to tinker with your equity structure, but there might be tax issues associated with that.
Another way to address it would be to have the startup execute some kind of note for the debt with you deferring payment until, e.g., the closing of your first round of financing, indicating that the note is to repay moneys already spent on IP development.
I suppose you could achieve the same effect by submitting a stack of receipts to your startup for reimbursement post-funding, but I'd be sure to discuss it with your investor(s) before you do that.