Purchase Stamps.com Postage

Purchase postage:

<?php
$s = new \RocketShipIt\Carrier\Stamps;
$resp = $s->purchasePostage(10.00);

Example error response:

stdClass Object
(
    [Authenticator] => exp=1465021666&uid=2736972&cty=swsim&ctk=sY+uyjAPvbFnJZGouxbtRd/XE=&iid=g949blZRUuaUT+DAW/Ygw==&rsid=0&irsid=3&ec=0&eacx=0&eac3=0&rrsid=0&raid=0&resellerid=0&pexp=0&pl=214748647&ast=1&ipaddr=101.170.255.117&ace=146538866&mac=FtMR3ldR4JYfv46GfIhGEOmo=
    [PurchaseStatus] => Rejected
    [TransactionID] => 0
    [PostageBalance] => stdClass Object
        (
            [AvailablePostage] => 497.9300
            [ControlTotal] => 770.00
        )

    [RejectionReason] => The total amount of postage in your account cannot exceed $500.00
    [MIRequired] =>
)

Example success response:

stdClass Object
(
    [Authenticator] => exp=1465021666&uid=2736972&cty=swsim&ctk=sY+uyjAPvbFnJZGouxbtRd/XE=&iid=g949blZRUuaUT+DAW/Ygw==&rsid=0&irsid=3&ec=0&eacx=0&eac3=0&rrsid=0&raid=0&resellerid=0&pexp=0&pl=214748647&ast=1&ipaddr=101.170.255.117&ace=146538866&mac=FtMR3ldR4JYfv46GfIhGEOmo=
    [PurchaseStatus] => Success
    [TransactionID] => 69611709
    [PostageBalance] => stdClass Object
        (
            [AvailablePostage] => 485.7600
            [ControlTotal] => 780.00
        )

    [MIRequired] =>
)