The free shipping wars are heating up again.

Amazon.com (amzn, +0.34%) has lowered its minimum order size for free-shipping to $25, the second reduction this year as its e-commerce battle with a resurgent Walmart (wmt, +0.92%) gets more intense.

Three months ago, Amazon lowered the minimum for shoppers who are not members of its Prime subscription program (which offers free two-day shipping for a $99 a year fee), meaning that now, the order minimum is about half the $49 it was in February.

The company did not officially announce the decrease, but its website currently says that online orders of at least $25 are eligible for free 5-8 days shipping on eligible items.

The Amazon move comes as Walmart, which has gotten more aggressive…