Hi,
All of the messages used in the shop are stored in the Shop.english.php file, in Themes/default/languages. If you want to change any of the messages, that's the place to do it (literally everything can be changed there).
The actual shop interface that the users see is in Shop.template.php. The bit you're talking about (the Per New Topic and stuff) looks like:
<div style="font-size: 0.85em; padding-top: 1ex;">', $txt['shop_welcome_full'], '
<br /><br />
<b>', $txt['shop_per_new_topic'], ': </b> ', $context['shop_newtopic'], '<br />
<b>', $txt['shop_per_new_post'], ': </b>', $context['shop_newpost'], '<br />
', $context['shop_bank_info'], '
<br />
<b>', $txt['shop_money_in_pocket'], ': </b>', $context['shop_money'], '<br />
', $context['shop_money_in_bank'], '
</div>
You'd probably want to edit it like:
<div style="font-size: 0.85em; padding-top: 1ex;">', $txt['shop_welcome_full'], '
<br /><br />
<b>', $txt['shop_per_new_topic'], ': </b> ', $context['shop_newtopic'], '<br />
<b>', $txt['shop_per_new_post'], ': </b>', $context['shop_newpost'], '<br />
', $context['shop_bank_info'], '
<br />
<b>', $txt['shop_money_in_pocket'], ': </b>', $context['shop_money'], '<br />
', $context['shop_money_in_bank'], '
Entering challenge: ', formatMoney(10), '<br />
Upload to Gallery: ', formatMoney(10), '<br />
</div>
formatMoney(10) writes the number 10, with the currency prefix and suffix added to it.
If you need any further help, please don't hesitate to ask!