How to center items in a collection-repeat in ionic?

I am using collection-repeat (from ionic framework) to display a list of envelopes in my app. Works great, but I would like to fine-tune the layout. Each envelope is 150px by 80px and it flows (so wider displays will have 3 or 4 or 5 columns).

I've calculated the amount of padding I need to add to the left side in order to center the grid, but I can't figure out how to update the DOM (I'd rather not use jQuery). Any suggestions?

Here is my HTML template:

<ion-content has-tabs="true">
  <div class="envelope-wrapper"
      collection-repeat="envelope in envelopes"
      collection-item-width="150"
      collection-item-height="80">
    <div class="envelope" style="background: {{ envelope.color }}">
      <div class="name">{{ envelope.name }}</div>
      <div class="amount">{{ envelope.amount | humanMoney }}</div>
    </div>
  </div>
</ion-content>

Here is my SCSS:

.envelope-wrapper {
  padding: 4px;
  height: 80px;
  width: 150px;

  .envelope {
    height: 100%;
    width: 100%;
    background: #eee;
    border: 1px solid rgba(0, 0, 0, .35);
    border-bottom: 1px solid rgba(0, 0, 0, .75);
    box-shadow: inset 0 1px 0 0 rgba(255, 255, 255, .5), 0 1px 3px #ccc;

    .name { ... }
    .amount { ... }
  }
}

And here is what it looks like when rendered. Notice all the white space on the right side. As I said, I've already calculated how wide that space is and I know the number of pixels I need to pad on the left, but I just can't figure out how to add left padding.