Taking the Add to Cart code example you provided above, this should work:
<form action="https://www.e-junkie.com/ecom/gb.php?c=cart&i=210879&cl=10022&ejc=2" target="ejejc" method="POST" accept-charset="UTF-8">
<input type="hidden" name="on0" value="Size"/>
Size:<br/>
<select name="os0">
<option value="Small">Small</option>
<option value="Medium">Medium</option>
<option value="Large">Large</option>
<option value="Extra Large">Extra Large</option>
</select>
<br/>
<a href=# onClick="javascript:return EJEJC_lc(this.parentNode);">ADD TO CART [T-SHIRT+MP3 $15.99]</a>
</form>
The only drawback to this, and there's really no way around it, is that buyers who have JavaScript disabled in their browser will be unable to Add to Cart this item. HTML provides no "standard" way to submit a form using a text link instead of a button, so the only way of doing it involves JavaScript tricks, which of course won't work if JavaScript has been disabled.